Democrat Tim Walz and Republican JD Vance clashed on Tuesday at a vice presidential debate that was surprisingly civil in the final stretch of an ugly election campaign marred by inflammatory rhetoric and two assassination attempts.

The two rivals, who have forcefully attacked each other on the campaign trail, mostly 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.

The most tense exchange occurred near the end of the debate, when Vance - who has said he would not have voted to certify the results of the 2020 election - avoided a question about whether he would challenge this year's vote if Trump loses.

Walz responded by blaming Trump's false claims of voter fraud for instigating the Jan. 6, 2021, mob that attacked the U.S. Capitol in an unsuccessful effort to prevent the certification of Joe Biden's 2020 election.

"He is still saying he didn't lose the election," Walz said, before turning to Vance. "Did he lose the 2020 election?"

Vance again sidestepped the question, instead accusing Harris of pursuing online censorship of opposing viewpoints.

"That is a damning non-answer," Walz said.

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

The rivals each sought to land a lasting blow at the last remaining debate before the Nov. 5 presidential election, arguing over the Middle East crisis, immigration, taxes, abortion, climate change and the economy.

But by and large the two men appeared intent on providing a demonstration of "Midwestern nice," thanking each other even while they went after their respective running mates in the traditional attack-dog role for vice presidential candidates.

Vance questioned why Harris had not done more to address inflation, immigration and the economy while serving in 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 prioritized 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 night's tone was a far cry from the divisiveness that has characterized the campaign. Trump has repeatedly denigrated Harris, including leveling racist and sexist attacks, and twice escaped attempts on his life. Walz had previously called his Republican opponents "weird," and Vance came under fire for past comments disparaging some Democrats as "childless cat ladies."