[embed]https://www.youtube.com/watch?v=kRh6598RmHM[/embed]

Vice-President Kamala Harris and former president Donald Trump are debating one another for the first —and potentially only — time in the U.S. presidential election campaign, sparring over the economy and reproductive rights.

The Democratic and Republican presidential nominees, who had previously never met, entered at the same time from opposite sides of the stage. They shook hands, with Harris offering her hand first and introducing herself by name.

The first exchange focused on the economy and the cost of living, one of the biggest issues of the campaign. Responding first, Harris spoke about her middle-class upbringing in California and said she wanted to create an "opportunity economy," while saying Trump would cut taxes for the richest Americans.

In his response, Trump defended his trade policy, saying that the Biden administration left his tariffs in place. He criticized Harris over inflation, said his new tariffs on foreign goods would be "substantial" and insisted plans to cut taxes would jump-start the economy. He also pivoted quickly to his top issue, immigration, claiming again without evidence that immigrants from "insane asylums" are crossing the U.S. southern border with Mexico.

The second question was about abortion and the reversal of Roe v. Wade, the landmark decision that established Americans' right to an abortion. Harris found footing in her response, saying "the government and Donald Trump certainly should not be telling a woman what to do with her body." 

Trump said he did the nation "a great service" by appointing the three Supreme Court justices who voted to overturn Roe v. Wade. Asked for a yes or no answer by ABC News anchor Linsey Davis, who is moderating the debate, Trump refused to rule out signing a national abortion ban.

Earlier, Davis corrected Trump's false claim that some states allow a baby to be killed after birth —  the first live fact-check of a candidate's onstage comments in either of this year's presidential debates.

The candidates took the stage in Philadelphia at 9 p.m. ET/6 p.m. PT. You can watch the livestream here on CBCNews.ca.

For Harris, 59, the debate is her best opportunity to capitalize on momentum she's built in the first seven weeks of her campaign. She will be looking to define her candidacy, introducing herself again to voters as a former prosecutor who understands the needs of the middle class and who could offer new hope to the nation and its fractured democracy. She will also have to distance herself from some of the Biden administration's less popular policies and present herself as a fresh leader.

For Trump, 78, the event is his best chance to try to undercut Harris's gains — which he's expected to try to achieve by painting Harris as a candidate incapable of addressing voters' concerns about crime, the economy and the border. He will likely continue his campaign messaging that has framed Harris as a "failed," "weak," and "dangerously liberal" incumbent from San Francisco who will bring Americans more of what they've seen from President Biden.

The stakes will be high for both candidates, but experts agree it will be a bigger night for Harris because she is the lesser known commodity. In her first presidential debate and longest unscripted event so far of the campaign, she will face an audience not only listening to her policies but watching how she handles a bombastic opponent who often speaks with little regard for facts.

Trump, with six presidential debates under his belt, already has a loyal base whose feelings haven't been swayed by two impeachments, several indictments and a felony conviction. Aides have reportedly cautioned the former president to stick to talking points related to policy, rather than continuing a pattern of personal attacks on Harris and crass attacks against women. 

ABC News anchors David Muir and Linsey Davis are moderating the 90-minute debate. As with the event in June, the candidates' microphones will be muted while the other is speaking and there is no in-studio audience.

The potential ramifications of a poor debate showing were made clear after Biden's halting performance during the previous debate on June 27, which led to his dropping from the race weeks later to make way for Harris. The saga overshadowed Trump's debate performance, during which he offered a litany of untruths and refused to condemn the Jan. 6, 2021, attack on the U.S. Capitol.

In an election that could again come down to tens of thousands of voters in just a few key states, even the tiniest shift in voter behaviour could change who wins the White House. Harris and Trump are effectively tied in the seven battleground states likely to decide the election, according to polling averages compiled by the New York Times.

Trump arrived in Philadelphia on Tuesday night, while Harris arrived Monday afternoon after preparing for the debate over several days at a downtown Pittsburgh hotel.

Earlier Tuesday, Biden said he spoken with Harris but declined to share the advice he gave. 

"She seems calm, cool and collected. I think she's going to do great," he told reporters before boarding Marine One outside the White House.

There are 56 days until U.S. voters head to the polls on Nov. 5. A vice-presidential debate is scheduled for Oct. 1.