The matchup is offering Americans their most detailed look at a campaign that's dramatically changed since the last debate in June. In rapid fashion, President Joe Biden bowed out of the race after his disastrous performance, Trump survived an assassination attempt and bothsides chose their running mates.

Harris, intent on demonstrating that she can press the Democratic case against Trump better than Biden did, walked up to Trump's lectern to introduce herself as the debate opened.

"Kamala Harris," she said, extending her hand to Trump, who received it in a handshake.

Trump, in turn, is trying to paint the vice president as an out-of-touch liberal while trying to win over voters skeptical he should return to the White House.

Trump, 78, has struggled to adapt to Harris, 59, who is the first woman, Black person and person of South Asian descent to serve as vice president. The Republican former president has at times resorted to invoking racial and gender stereotypes, frustrating allies who want Trump to focus instead on policy differences with Harris.

The vice president, for her part, is trying to claim a share of credit for the Biden administration's accomplishments while also addressing its low moments and explaining her shifts away from more liberal positions she took in the past.

The debate is subjecting Harris, who has sat for only a single formal interview in the past six weeks, to a rare moment of sustained questioning.

Her running mate, Minnesota Gov. Tim Walz, speaking Tuesday to donors in Las Vegas, highlighted Trump's experience debating after three White House runs.

"No one in modern times has done more of these," he said. "The good news is that this is his seventh debate, and we know exactly what to expect."

The first early ballots of the presidential race will go out just hours after the debate, hosted by ABC News. Absentee ballots are set to be sent out beginning Wednesday in Alabama.

The candidates met in a small, blue-lit amphitheater converted into a television studio, with no live audience, meaning there would be no rowdy applause, cheers or jeers.

The intimate setting -- with the candidates' lecterns positioned less than 10 feet from each other -- belied the contentious debate to follow.

Trump and his campaign have spotlighted far-left positions Harris took during her failed 2020 presidential bid. He had been assisted in his informal debate prep sessions by Tulsi Gabbard, the former Democratic congresswoman and presidential candidate who tore into Harris during their primary debates.

Harris has sought to defend her shifts away from liberal causes to more moderate stances on fracking, expanding Medicare for all and mandatory gun buyback programs -- and even backing away from her position that plastic straws should be banned -- as pragmatism, insisting that her "values remain the same." Her campaign on Monday published a page on its website listing her positions on key issues.

The vice president, who has been the Biden administration's most outspoken supporter of abortion access after the Supreme Court overturned Roe v. Wade in 2022, was expected to focus on calling out Trump's inconsistencies around women's reproductive care, including his announcement that he will vote to protect Florida's six-week abortion ban in a statewide referendum this fall.

She is likely to warn that Trump presents a threat to democracy, from his attempts in 2020 to overturn his loss in the presidential election, spurring his angry supporters to attack the U.S. Capitol on Jan. 6, 2021, through comments he made as recently as last weekend. Trump on social media issued yet another message of retribution, threatening that if he wins he will jail "those involved in unscrupulous behavior," including lawyers, political operatives, donors, voters and election officials.

While Tuesday's meeting might be the last time the candidates cross paths on the debate stage, they may cross paths again Wednesday when they both mark the 23rd anniversary of the 9/11 attacks. Harris will join Biden

Harris, Trump and Biden plan to all be at ground zero in lower Manhattan and the Flight 93 National Memorial near Shanksville, Pennsylvania on Wednesday. Harris and Biden will also visit the Pentagon in Arlington, Virginia later in the day for a ceremony there.

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Price and Miller reported from Washington. AP Polling Editor Amelia Thomson-DeVeaux in Washington and Thomas Beaumont in Las Vegas, Bill Barrow in Atlanta contributed to this report.