WASHINGTON, Oct 1, 2024 (BSS/AFP) - 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 the decisive votes of the American heartland.

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

Trump has refused a second clash with Harris, meaning that Tuesday's debate in New York 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 that could decide an election that remains on a knife-edge with just five weeks to go.

History suggests that 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 campaign has seen Vance and Trump use increasingly divisive rhetoric -- and even falsely accuse migrants of eating people's pets -- meaning that the debate is almost guaranteed to make for fiery television.

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

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

Both are military veterans with strong blue-collar credentials, with Vance known as the author of the Rust Belt memoir "Hillbilly Elegy" and Walz boasting a folksy persona as a former teacher and football coach.

But 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 the false claims that Haitian migrants in an Ohio town ate pet cats and dogs.

- 'High drama' -

His goal will be to overcome polls that initially had him as one of the least popular vice presidential 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 aired Monday on Fox Nation.

Vance, who served as a US Marine in Iraq, is also expected to hit Walz over claims that the Democrat left his US National Guard unit before it deployed to Iraq.

Televised debates have already proved their ability to shock this year, with Biden forced to drop his bid for a second term after a disastrous performance against Trump in June.

Whalen said the United States had "barely had a vice presidential debate that's had any appreciable difference" in the past, but that Tuesday's clash could produce "high drama" for viewers who love political theater.

Trump -- who was widely considered to have lost his September 10 debate with Harris -- said he would be live-commenting on the debate between the "brilliant J.D. Vance" and the "highly inarticulate" Walz.

Both candidates have been preparing intensively, with Walz using Transportation Secretary Pete Buttigieg to stand in for Vance.

Their clash could feature a little extra spice thanks to the fact that Vance's and Walz's microphones will be live throughout, allowing them to cut in on their rivals.

There will, however, be no studio audience for the debate, which is being moderated by CBS hosts Norah O'Donnell and Margaret Brennan.