Donald Trump is sweeping all before him in the race for the Republican presidential nomination, but the primary votes have unveiled some worrying obstacles in his path to a second term in the White House.

Despite having a virtual lock on the nomination ahead of next week's 15-state "Super Tuesday" voting bonanza, the 77-year-old has been hemorrhaging moderate Republicans, who have signaled they'd prefer someone else -- anyone else -- in the Oval Office.

His double-digit victories over Nikki Haley in the early voting states have also obscured flashing red lights over his standing with the independents he'll need to prevail against President Joe Biden in November.

In New Hampshire, independents broke two-to-one for Haley, and she bested Trump among South Carolina's suburban voters, a key constituency that will likely hold the keys to the White House.

Some 40 percent of her supporters in the Palmetto State, where she was governor for six years, said in exit polls that they were opposed to Trump.

Alyssa Farah Griffin, a communications director in the Trump White House, warned that Haley's vote share was a "five-alarm fire" for her former boss.

"Somebody who's running as virtually an incumbent -- Donald Trump -- getting 60 percent, and 40 percent being against him? That's not a mandate," she said on a CNN discussion panel.

- 'Trump has a problem' -

Trump waved away Haley's 43 percent share in New Hampshire, pointing to regulations allowing for mass participation by non-Republicans in a state deemed the former UN ambassador's best chance for success.

But he had no such excuse in deeply conservative South Carolina, where pollsters found that just five percent of voters in the Republican primary identified as Democrats. Meanwhile almost half of Republican caucus-goers backed other candidates in deeply conservative Iowa.

"Donald Trump has a problem, whether he wants to admit it or not," Haley said in a statement putting a gloss on her 42-point defeat to Trump in the latest primary in Michigan on Tuesday.

"Forty percent of the Republican primary electorate wants nothing to do with him, and he is doing absolutely nothing to bring them into his increasingly shrinking tent."

It's Haley's job to talk down Trump's appeal and he expects to win back some of her support, particularly the 23 percent in South Carolina who said in exit polls that they would not be upset if he were reelected.

Trump backers have also pointed to Biden's poor polling with independents, including a recent NBC survey that gave him a 27 percent approval among the group.

Charlie Kolean, the chief strategist at conservative political consultancy RED PAC, points to a new Morning Consult poll giving Trump leads over Biden of two to nine points across the seven swing states where elections are won and lost.

"Trump is well positioned to win the general election. He focuses on issues that clearly matter to voters and win at the ballot box -- government transparency, fiscal responsibility, energy independence and jobs," he told AFP.

- 'Happy talk' -

But the Biden campaign is banking on some of the swing state races narrowing as voters reacquaint themselves with Trump, get used to the idea of the former president as a nominee again and catch up with his multiple criminal indictments.

Keith Nahigian, a veteran of six presidential campaigns and former member of Trump's transition team, believes the embattled tycoon's legal woes, rather than his primary performances, are the biggest threat to his reelection.

"The most valuable thing on a campaign is time -- time to raise money, time to meet people, time to travel to media markets -- and anything that steals from that could be harmful," he told AFP.

"However the opposite effect has been occurring so far in this cycle as the news around his indictments has rallied supporters."

Bill Kristol, a former chief of staff to Republican vice president Dan Quayle and a Trump critic, says that if the ex-president were able to win back two-thirds of Haley's supporters he'd likely end up with around 92 percent among Republicans -- a repeat of his 2020 performance.

"(That) was enough to elect Biden, but barely. People who are glorying at the fact that he's only at 59 or was only at 55 in New Hampshire -- that's a little too much happy talk," Kristol said on a podcast for center-right publication The Bulwark.

"But the people are saying that there's no resistance to speak of in Republican Party, that's too pessimistic."