Republican presidential candidates Arizona Senator John McCain (L) and Texas Governor George W. Bush (R) make points to moderator Larry King (C) as Alan Keyes (partially obscured) looks on during a televised Republican presidential debate on the CNN network in Columbia, February 15. The South Carolina Republican primary election is February 19.

US Republican presidential candidate Senator John McCain (R-AZ) stands with Major Gordon Hilbun (R), McCain's vehicle commander in Haditha, Iraq, on March 16, 2008. Picture taken March 16, 2008.

Vietnam war veteran and former Republican presidential frontrunner, Senator John McCain of Arizona steps down a dark corridor separating jail cells with his son Jack during a tour of the infamous "Hanoi Hilton" jail April 26 where he spent time as a prisoner of war. During the visit McCain said that he could not forgive the jailers who mistreated and killed his comrades. A quarter century after the communist victory in Vietnam, McCain and other U.S. veterans have arrived in Vietnam to mark the April 30, 1975 ending of the war. McCain's combat plane was shot down over the lake and he spent five years as a prisoner of war during the conflict.

Republican presidential candidate Arizona Sen. John McCain, his wife Cindy, (R) and daughter Bridget, (bottom) celebrate with supporters on the evening of the Super Tuesday primaries in Los Angeles March 7. McCain won primaries in Vermont, Connecticut, Massachussets and Rhode Island, but lost the big states of New York and California.

Senators including John McCain and Bernie Sanders arrive for the inauguration ceremonies to swear in Donald Trump as the 45th president of the United States at the U.S. Capitol in Washington, U.S., January 20, 2017.

US Republican presidential candidate Senator John McCain (R-AZ) and his running mate Alaska Governor Sarah Palin hug during a campaign stop in O'Fallon, Missouri August 31, 2008.

U.S. Senator John McCain (R-AZ) (C) gives a tour of the Rotunda to Vietnam's Vice Minister of Defense Lieutenant General Do Ba Ty (2nd R) and his entourage at the U.S. Capitol in Washington, June 20, 2013. McCain, who was shot down over Hanoi in 1967, is a former prisoner of war in North Vietnam and one of the country's most prominent Vietnam veterans.

U.S. Republican presidential nominee Senator John McCain (R-AZ), along with his wife Cindy, gives a thumbs up to the crowd after his concession speech to President-elect Barack Obama in Phoenix November 4, 2008.

(L-R) Former Arkansas Governor Mike Huckabee, Republican presidential candidates former New York City Mayor Rudy Giuliani, and U.S. Sen. John McCain (R-AZ) take the stage for Fox News Channel Republican Presidential debate at the University of New Hampshire in Durham, New Hampshire September 5, 2007.

U.S. Republican presidential candidate Senator John McCain (center) (R-AZ) arrives at the Western Wall, Judaism's holiest prayer site, in Jerusalem's Old City March 19, 2008. McCain came to Israel on Tuesday as part of a fact-finding tour of the Middle East which could also boost his popularity among American Jewish voters.

(L-R, in mirror) U.S. Senators Lindsey Graham (R-SC) , Kelly Ayotte (R-NH) and John McCain (R-AZ) arrive for the start of a news conference to talk about new legislation to restrict prisoner transfers from the detention center at Guantanamo Bay, at the U.S. Capitol in Washington January 13, 2015. Several U.S. Republican senators on Tuesday proposed legislation that would place a moratorium on the release of most of the prisoners held at Guantanamo Bay, Cuba, saying they pose too much danger to the United States and its allies.

President George Bush addresses reporters at the White House, October 23, on the release of documents by Hanoi on Americans missing in Southeast Asia. Bush, with Senator John McCain (L) and Senator John Kerry, said the information will allow the United States to "begin writing the last chapter of the Vietnam War."

U.S. Republican presidential candidate and U.S. Senator Lindsey Graham (R), with U.S. Senator John McCain (2nd L) at his side, places his bet at the roulette table at the Manchester Bingo Center and Poker Room in Manchester, New Hampshire October 9, 2015.

U.S. Navy Lt. Comdr. John S. McCain is interviewed about his experiences as a prisoner of war during the war in Vietnam, April 24, 1973. Library of Congress/Thomas J. O'Halloran/Handout via REUTERS

U.S. President Ronald Reagan meeting with Republican senatorial candidate John McCain ahead of McCain's 1986 election campaign, in the Oval Office of the White House.

US Republican presidential nominee Senator John McCain (R-AZ) reacts to almost heading the wrong way off the stage after shaking hands with Democratic presidential nominee Senator Barack Obama (D-IL) at the conclusion of the final presidential debate at Hofstra University in Hempstead, New York, October 15, 2008.

U.S. Republican presidential nominee Senator John McCain (R-AZ) listens as he is introduced at a campaign rally in Fayetteville, North Carolina October 28, 2008. REUTERS/Brian Snyder

U.S. Republican presidential nominee Senator John McCain (R-AZ) stands with his wife Cindy as he delivers his concession speech after US Sen. Barack Obama (D-IL) defeated him during his election night rally in Phoenix November 4, 2008.

A U.S. soldier watches U.S. Senator John McCain (R-AZ) on television as he gives a speech after losing the US general election, in Bagram airbase north of Kabul November 5, 2008.

Republican presidential candidate US Senator John McCain's bus drives down 42nd Street to a campaign stop in New York City, February 4, 2008.

US Republican presidential candidate Senator John McCain (R-AZ) delivers remarks at the U.S. Naval Academy in Annapolis, Maryland, April 2, 2008. McCain spent four years at the U.S. Naval Academy prior to training as a fighter pilot and heading to Vietnam, where his plane was shot down and he spent 5-1/2 years as a prisoner of war.

Republican presidential candidate Texas Gov. George W. Bush (L) and vanquished contender Sen. John McCain (R-AZ) joke as they answer questions from the media during a press conference in Pittsburgh May 9. McCain endorsed his former political rival Texas Gov. Bush as the Republican U.S. presidential nominee, but said he would not be Bush's running-mate.

Arizona Senator and Republican presidental hopeful John McCain looks up while speaking with his campaign manager Rick Davis (L) and national political director John Weaver during a flight from South Carolina to California February 4. Recent polls indicate that the lead held by Texas Governor George Bush has eroded and the two candidates are now locked in a dead heat in South Carolina.

Senator John McCain (R-AZ) speaks to reporters as he arrives for a vote on Capitol Hill in Washington, U.S., October 18, 2017.

Republican presidential nominee Senator John McCain (R-AZ) (R) and his wife Cindy (2nd R) stand with vice presidential nominee Alaska Governor Sarah Palin and her husband Todd after McCain accepted the presidential nomination at the 2008 Republican National Convention in St. Paul, Minnesota September 4, 2008.

Republican presidential candidate Senator John McCain embraces a war veteran as confetti falls during his 100th town hall style campaign event in Peterborough , New Hampshire, January 5, 2008.

The Defiance High School Band waits to greet U.S. Republican presidential nominee Senator John McCain (R-AZ) (far L) and his wife Cindy in Defiance, Ohio October 30, 2008. McCain is on a two-day campaign bus tour through the state of Ohio.

Republican presidential candidate U.S. Senator John McCain (R-AZ) leaves after a campaign rally at the convention center of Tampa, Florida January 28, 2008.

Republican presidential candidate Senator John McCain (R-AZ) arrives for a news conference in Annapolis, Maryland, February 11, 2008.

U.S. Senator John McCain (R-AZ) and former Alaska Governor and vice presidential candidate Sarah Palin acknowledge the crowd during a campaign rally for McCain at the Pima County Fairgrounds in Tucson, Arizona March 26, 2010. McCain, who is seeking a fifth term as U.S. Senator, received Palin's endorsement and will challenge JD Hayworth during the Republican primary in August 2010.

Republican presidential candidate and former Massachusetts Governor Mitt Romney (L) and U.S. Senator John McCain (R-AZ) talk on Romney's campaign bus enroute to Peterborough, New Hampshire January 4, 2012, one day after Romney won the Iowa caucus.

Peterborough, United States of AmericaConfetti fills the Peterborough town hall as Republican presidential hopeful John McCain and his wife Cindy leave the stage after McCain addressed residents here January 30. George W. Bush has pulled into a statistical dead heat with McCain two days before the New Hampshire Republican primary, according to a Reuters-WHDH daily tracking poll released on Sunday.

John McCain (Bottom R) poses with his U.S. Navy squadron in 1965. National Archives/Handout via REUTERS.

U.S. President Richard Nixon greets former Vietnam prisoner of war John McCain at a pre-POW dinner reception in Washington, May 24, 1973.

U.S. Republican presidential nominee Senator John McCain (R-AZ), with his wife Cindy, speaks at a campaign rally at St. Anselm College in Manchester, New Hampshire October 22, 2008.

U.S. Republican presidential nominee Senator John McCain (R-AZ) speaks at a campaign rally in Defiance, Ohio October 30, 2008. McCain is on a two-day campaign bus tour through the state of Ohio.

Presidential hopeful U.S. Senator John McCain (R-AZ) (R) speaks with U.S. Army Master Sergeant Daniel R. Robles, (L), who was wounded in Iraq, before the ceremony to announce the opening of The Center for the Intrepid in San Antonio, Texas January 29, 2007. The Center for the Intrepid will provide injured service members with occupational and physical therapy to help with their rehabilitation.

U.S. President George W. Bush and presumptive Republican presidential nominee Sen. John McCain walk toward the Oval Office of the White House in Washington March 5, 2008. McCain scored victories in Texas, Ohio, Vermont and Rhode Island on Tuesday to complete his improbable comeback from the political graveyard last summer to become his party's standard-bearer.

Vietnam war veteran and former Republican presidential frontrunner, Senator John McCain of Arizona walks past the iron bars of a jail cell within the infamous "Hanoi Hilton" jail April 26 on a tour of the former prison in which he spent time as a prisoner of war. During the visit McCain said that he could not forgive the jailers who mistreated and killed his comrades. A quarter century after the communist victory in Vietnam, McCain and other U.S. veterans have arrived in Vietnam to mark the April 30, 1975 ending of the war. McCain's combat plane was shot down over the lake and he spent five years as a prisoner of war during the conflict. Date changed

U.S. President-elect Barack Obama speaks with U.S. Senator John McCain (R-AZ) at a bipartisan dinner honoring McCain in Washington, DC January 19, 2009.

U.S. Senator John McCain (R-AZ) speaks after being awarded the 2017 Liberty Medal by former U.S. Vice President Joe Biden (unseen) at the Independence Hall in Philadelphia, Pennsylvania, U.S., October 16, 2017.

A still image from video shows U.S. Senator John McCain (R-AZ), who had been recuperating in Arizona after being diagnosed with brain cancer, acknowledging applause as he arrives on the floor of the U.S. Senate after returning to Washington for a vote on healthcare reform in Washington, U.S., July 25, 2017. SENATE TV/Handout via REUTERS

Senator John McCain (R-AZ), recently diagnosed with an aggressive form of brain cancer, departs after returning to the Senate to vote on health care legislation on Capitol Hill in Washington, U.S., July 25, 2017.

Democratic presidential candidate Senator Barack Obama (D-IL) (back) and Republican Presidential candidate Senator John McCain(R-AZ) embrace as they appear together at a forum on national service at Columbia University in New York, September 11, 2008.

U.S. Senator John McCain (R-AZ) (C) talks to reporters on his way to vote on the Senate floor at the U.S. Capitol in Washington, October 12, 2013.

U.S. Republican presidential nominee Senator John McCain (R-AZ) (R) hugs his wife Cindy at an airport hangar campaign rally in Indianapolis, Indiana November 3, 2008. Senator McCain is scheduled to make campaign appearances in seven states on the final day before the election.

Republican presidential candidate Arizona Senator John McCain (R) listens to advice from his campaign staff onboard his bus as his wife Cindy (L) looks on near Concord, New Hampshire January 25. Senator McCain did not compete in the Iowa caucus yesterday, instead concentrating his resources for New Hampshire's first-in-the-nation primary February 1.

Republican presidential candidate U.S. Senator John McCain (R-AZ) smiles as talks to journalists on a bus after a campaign event in West Palm Beach, Florida, January 24, 2008.

US Republican presidential candidate Senator John McCain (R-AZ) unbuckles his flight helmet as he arrives at Sather Air Base in Baghdad, March 16, 2008. Picture taken March 16, 2008.

Set against the dramatic landscape of Sedona, Arizona Senator John McCain looks down while announcing his withdrawal from the Republican presidential campaign March 9 following disappointing results on Super Tuesday. McCain, a former Navy pilot and a prisoner of war in Vietnam for 5 1/2 years, said he was suspending his campaign so that he could take some time with his wife Cindy, (L) to reflect on their recent experiences and decide how best to serve the country.

Sen. John McCain (R-AZ) arrives on Capitol Hill in Washington, U.S. July 25, 2017.

Sen. John McCain heads to the Senate floor ahead of votes on Capitol Hill in Washington, U.S., December 6, 2017.

Russian opposition leader Vladimir Kara-Murza, vice chairman of Open Russia, and Senator John McCain (R-AZ) prepare to testify before a Senate Appropriations State, Foreign Operations and Related Programs Subcommittee hearing on "Civil Society Perspectives on Russia" on Capitol Hill in Washington, U.S., March 29, 2017.

U.S. Republican presidential nominee Senator John McCain (R-AZ) (L) and U.S. Senator Joe Lieberman (I-CT) point to a sign in the crowd at a campaign rally in Grand Junction, Colorado November 4, 2008, the day of the U.S. presidential election.