NEW YORK: US vice presidential contenders J.D. Vance and Tim Walz face off in what could be an unusually important undercard debate Tuesday as they compete for decisive votes in America's heartland just weeks before the election.

The showdown between Walz, the Democratic Minnesota governor chosen by Kamala Harris, and Vance, the Republican senator from Ohio who is Donald Trump's running mate, is likely to be the last of the 2024 campaign.

Trump has refused a second debate with Vice President Harris, meaning this could be the final chance to see the two tickets go head to head.

Vance, 40, and Walz, 60, each claim to be the true voice of the crucial Midwestern swing states – including Michigan and Wisconsin – that could decide an election that remains on a knife-edge with five weeks to go.

Both have been out of the spotlight as of late, but Walz offered a brief message on X: "Looking forward to tonight."

History suggests vice presidential debates rarely move the dial much. But in an election campaign that has seen Harris step in for US President Joe Biden unprecedentedly late in the game, Tuesday's contest may have added significance.

The race has seen Vance and Trump use increasingly divisive rhetoric and even falsely accuse immigrants of eating people's pets – meaning the debate is almost guaranteed to make for fiery television.

"It will whet a lot of people's appetites for Nov 5," Thomas Whalen, an associate professor of social sciences at Boston University, told AFP.

But the debate itself risked being overshadowed by Mideast tensions, after Iran launched ballistic missiles against Israel, which said it largely repelled the attack.

Trump, visiting swing state Wisconsin on Tuesday, made zero mention of Vance or the VP debate in a rambling address. But he did insist that "if I were in charge, today's attack on Israel never would have happened."

Should Harris and Walz win, he warned, "the world goes up in smoke."

Harris for her part pledged her "unwavering" commitment to the security of Israel after Iran launched what she called "a reckless and brazen attack" on America's ally.

She said she "fully" supported Biden's order for the US military to shoot down some of the missiles.

The CBS clash also comes as several states dig out from enormous storm Helene, which has left at least 130 people dead and brought misery to thousands more.

The devastation, and how to address such disasters, is likely to come up during the debate.

Walz and Vance were each picked by their bosses to reach out to voters in the Midwestern battlegrounds where, thanks to the country's idiosyncratic electoral college system, a few thousand votes could determine who wins the White House race.

Both are military veterans with strong blue-collar credentials. Vance authored the Rust Belt memoir "Hillbilly Elegy" while Walz boasts a folksy persona as a former teacher and football coach.

The similarities end there.

The combative Vance shares Trump's penchant for courting controversy, whether by smearing Democrats as "childless cat ladies" or by boosting false claims that Haitians living in an Ohio town ate residents' pets.

His goal will be to overcome polls that initially had him as one of the least popular VP nominees in history, after a series of previous comments on women and abortion were unearthed.

"Vance has to be careful, because I think a trap has been laid for him," said Whalen.

The cheery Walz will be seeking to introduce himself to a public that barely knows him, after Harris's swift rise to replace Biden as the Democratic nominee.

He became a hit with Democrats for branding Vance and Trump "weird" and for his progressive politics – but that will be a target for Vance as he and Trump seek to paint Walz and Harris as "Marxists."

Vance "is going up against a moron, a total moron," Trump said in an interview Monday on Fox Nation.

Televised debates have already proved their ability to shock this year, with Biden forced to drop his reelection bid after a disastrous performance against Trump in June brought long-simmering concerns about his age to the fore.

Whalen said few vice presidential debates have "had any appreciable difference" in the past, but Tuesday's clash could produce "high drama" for viewers who love political theater.

  - AFP