With its war zone imagery and macho voiceover it could be a movie trailer -- but the sunglasses-clad star of a new campaign ad is none other than 80-year-old US President Joe Biden.

Facing a battle in the opinion polls, America's oldest ever commander-in-chief is using his surprise trip to Kyiv in February to sell himself as a "true leader" ahead of the 2024 election.

The 60-second spot -- which debuted on social media on Thursday and will air on TV in US battleground states during prime time on Saturday -- draws a stark contrast with his Republican rivals over the Ukraine war.

The message is simple: the Democrat's support against Russia's brutal 2022 invasion shows global leadership, while Republicans including Donald Trump have wobbled all over the place on the issue.

It was a political gamble for Biden to give Kyiv his full backing -- and more than $43 billion in military aid -- but he is now doubling down that it could help him at the ballot box.

"It was the first time in modern history that an American president went into a war zone not controlled by the United States," the ad starts, narrated by a gravelly voiced man.

Airing as Biden attends the G20 summit in India -- which Russian President Vladimir Putin is skipping -- the ad shows photos of Biden taking a train from Poland to Kyiv "under cover of night."

It then describes him walking "shoulder to shoulder with our allies in the war-torn streets, standing up for democracy in a place where a tyrant is waging war to take it away."

Cue a picture of Putin raising a glass of champagne. Then footage of soldiers firing cannons, followed by Biden in his trademark aviator sunglasses meeting Ukrainian President Volodymyr Zelensky.

"In the middle of a war zone, Joe Biden showed the world what America is made of. That's the quiet strength of a true leader who doesn't back down to a dictator," the ad concludes.

- 'American leadership' -


Quietness is part of the problem for Biden, though.

Biden is struggling with tepid approval ratings and low visibility ahead of a possible rematch with airtime-hogging Trump next year.

And literally too. Reporters sometimes strain to hear the octogenarian -- yet more fuel for concerns among Republican and Democrat voters alike that he's too old for a second term.

But while foreign policy is rarely a key theme in US elections, the ad shows Team Biden sees his Ukraine stance as crucial for a counteroffensive against Republican contenders.

It's also airing as the US Congress is to debate whether to keep funding Ukraine, with some Republicans threatening to block the latest tranche.

Biden-Harris campaign manager Julie Chavez Rodriguez said in a statement that Biden had "restored true American leadership on the world stage."

"While the MAGA (Make America Great Again) Republicans running for president have sided with dictators over democracy, President Biden rallied the West," she said.

Republican candidate Ron DeSantis stumbled earlier this year when he downplayed Russia's invasion as a "territorial dispute". Upstart rival Vivek Ramaswamy has called on Ukraine to make "major concessions" to Moscow.

Then there is Trump. The 77-year-old former president has claimed he could end the war in 24 hours and declined to say who he wants to win, while his frequent statements of praise and admiration for Putin have long raised eyebrows.

Many Republican voters appear increasingly Ukraine-skeptic, swayed by messaging in the right wing of the party and conservative media that US funds should go to fighting illegal immigration, not helping Ukraine.

Unproven allegations about the business dealings of Biden's son Hunter in Ukraine have also fed into growing lack of support on the right for the pro-Western government in Kyiv.

With some polls showing him now neck-and-neck with Trump, Biden has clearly decided it is time for the big guns.

"I'm Joe Biden and I approve this message," he says at the end of the ad.