Democrat Tim Walz and Republican JD Vance clashed over the Middle East crisis, immigration, taxes, abortion, climate change, and the economy on Tuesday at a vice presidential debate that was heavy on policy disagreements but light on personal attacks.

The two rivals, who have savaged each other on the campaign trail, struck a cordial tone, instead saving their fire for the candidates at the top of their tickets, Democratic Vice President Kamala Harris and Republican former President Donald Trump.

Vance questioned why Harris had not done more to address inflation, immigration, and the economy while serving in President Joe Biden's administration, mounting a consistent attack line that Trump often failed to deliver while debating Harris last month.

"If Kamala Harris has such great plans for how to address middle-class problems, then she ought to do them now - not when asking for promotion, but in the job the American people gave her 3-1/2 years ago," Vance said.

Walz described Trump as an unstable leader who had prioritised billionaires and turned Vance's criticism on its head on the issue of immigration, attacking Trump for pressuring Republicans in Congress to abandon a bipartisan border security bill earlier this year.

"Most of us want to solve this," Walz said of immigration. "Donald Trump had four years to do this, and he promised you, Americans, how easy it will be."

The debate at the CBS Broadcast Center in New York began with the escalating crisis in the Middle East, after Israel continued its assault on southern Lebanon on Tuesday and Iran mounted retaliatory missiles strikes against Israel.

Walz said Trump is too "fickle" and sympathetic to strongmen to be trusted to handle the growing conflict, while Vance asserted that Trump had made the world more secure during his term.

Asked whether he would support a preemptive strike against Iran by Israel, Vance suggested he would defer to Israel's judgment, while Walz did not directly answer the question.

Walz, 60, the liberal governor of Minnesota and a former high school teacher, and Vance, 40, a bestselling author and conservative firebrand US senator from Ohio, have portrayed themselves as two sons of America's Midwestern heartland with deeply opposing views on the issues gripping the country.

Trump, watching on television, was posting furiously during the debate, sometimes twice a minute, on his Truth Social site.

Seeking a moment

The rivals each sought to land a lasting blow at the last remaining debate before the Nov. 5 presidential election.

Political analysts say vice presidential debates can be fiery but generally do not alter the outcome of an election. That said, even a slight shift in public opinion could prove decisive with the race on a razor's edge five weeks before Election Day.

Walz was asked about a report this week that he was not in China during the violent 1989 Tiananmen Square crackdown, as he had previously claimed.

"I'm a knucklehead at times," he said. "I got there that summer and misspoke on this. So I was in Hong Kong and China during the democracy protests, and from that I learned a lot about what it means to be in governance."

Vance, meanwhile, defended his running mate despite having criticized Trump ahead of the 2016 election.

"I was wrong about Donald Trump," he said. "I was wrong, first of all, because I believe some of the media stories that turned out to be dishonest fabrications of his record. But most importantly, Donald Trump delivered for the American people."

Walz also criticised Trump for his role in appointing three US Supreme Court justices who joined the court's decision to eliminate a nearly half-century nationwide right to abortion, an issue that has proven damaging to Republicans.

"Donald Trump put this all into motion," Walz said. "He brags about how great it was that he put the judges in and overturned Roe versus Wade," the 1972 Supreme Court decision protecting a right to abortion.

Vance, known for his deeply conservative stance on abortion, struck a more moderate tone on Tuesday, saying he did not back a national ban despite having previously done so. He said Trump's view is that individual states should decide whether to limit abortion.

In a social media post, Trump said he would veto a national ban, weeks after he refused to say whether he would during the presidential debate.

Despite Vance's having written "Hillbilly Elegy," a popular 2016 memoir, US voters have a negative view of him, Reuters/Ipsos polling shows, with 51% of registered voters saying they view him unfavourably, compared with 39% who view him favourably. Meanwhile Walz was viewed favourably by 44% of registered voters, with 43% reporting an unfavourable view in the Sept. 20-23 poll.

Harris was widely viewed as the winner of her sole debate with Trump on Sept. 10 in Philadelphia, which was watched by an estimated 67 million people and was far more chaotic than Tuesday's affair.

That square-off did little to change the trajectory of an extremely close election battle. While Harris has edged ahead in national polls, most surveys show voters remain fairly evenly divided in the seven states that will decide the November election.