Washington DC - President Joe Biden is banking on his support for Ukraine in its war with Russia to be a vote-winner with a new campaign ad set to air on TV in key battleground states during prime time on Saturday.

Facing a battle in the opinion polls, Biden 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 – 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.

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"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's narrator says.

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, along with the traditional seal of approval from the candidate.

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.

Florida Governor 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.

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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.

Republicans have become increasingly skeptical of support for Ukraine, with over two thirds of those polled recently saying Congress should not authorize new funding.