PHILADELPHIA — Vice President Kamala Harris sought to take down former President Donald Trump at their first meeting on the debate stage with the help of ABC News moderators who frequently fact checked Mr. Trump while letting Ms. Harris largely get away with debunked claims against him while providing only vague answers on how she would govern as president.

The two squared off on the economy, abortion, immigration and the wars in Ukraine and Israel.

Ms. Harris mostly passed up opportunities to explain why she has pivoted on a half-dozen major issues and instead pushed her plan to give families a $6,000 child tax credit and small businesses $50,000 in startup funds.

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She spent much of the debate attacking Mr. Trump aggressively as a weak, divisive leader and often accused him of lying, with the help of ABC News moderators who frequently did “fact checks” on Mr. Trump, sometimes incorrectly, and mostly gave Ms. Harris a pass.

Ms. Harris, for example, was asked whether she bore any responsibility for the disastrous U.S. withdrawal from Afghanistan, which left 13 service members dead from a terrorist suicide bomber.

Instead of answering directly, she blamed Mr. Trump for striking a “weak, terrible deal” with the Taliban before President Biden took office.

Even though Ms. Harris said she agreed with Mr. Biden on the plan to withdraw from Afghanistan, there was no follow-up question from moderators David Muir or Linsey Davis about how she viewed the calamitous and deadly execution of the pullout under his orders.

Mr. Trump touted the successes of his presidency, chiefly the strong economy that prevailed before the COVID-19 pandemic struck in March 2021, and the quiet world stage, which since his departure has erupted into a Russian invasion of Ukraine, a war between Israel and Hamas terrorists in the Gaza Strip and an emboldened Iran and China.

Mr. Trump made grand promises, including a pledge to settle the war between Ukraine and Russia before he enters the White House if he is elected president.

“That is a war that is dying to be settled,” said Mr. Trump, blaming the Biden administration for failing to even pick up the phone to try to talk with Russian President Vladimir Putin.

He also sought to tie Ms. Harris to the unpopular president, who left the race because of sinking poll numbers and questions about his cognitive fitness.

Mr. Trump linked Ms. Harris to Mr. Biden on several occasions, saying, “Biden and you” and “she and her boss,” and repeatedly referring to Mr. Biden and Ms. Harris as “they.”

“She is Biden,” Mr. Trump said. “The worst inflation we’ve ever had, a horrible economy because inflation has made it so bad. She can’t get away with that.”

Ms. Harris responded, “Clearly, I’m not Joe Biden, and certainly, I’m not Donald Trump.”

The vice president sought to tie the two men together as an old generation of politicians who needed to be replaced.

“What I do offer is a new generation of leadership for our country, one who believes in what is possible, one who brings a sense of optimism about what we can do, instead of always disparaging the American people,” she said.

Ms. Harris explained only one flip-flop in any detail: her reversal on a 2019 pledge to ban fracking for natural gas on federal land.

“My values have not changed,” Ms. Harris said. “My position is we have got to invest in diverse sources of energy so we reduce our reliance on foreign oil. We have had the largest increase of domestic oil production in history.”

Mr. Trump seized on her answer, pointing out she has suddenly jettisoned other important policy positions now that she is running for president, including defunding the police and ordering gun buy-backs.

“Fracking? She’s been against it for 12 years,” Mr. Trump said after Ms. Harris denied supporting a ban.

“She wants to confiscate your guns, and she will never allow fracking in Pennsylvania,” he said.

Ms. Harris frequently, and successfully, goaded Mr. Trump, by attacking him on his criminal convictions and looming criminal cases, and by claiming that people were leaving Mr. Trump’s rallies early out of boredom.

Mr. Trump would at least briefly take the bait, but he would often pivot back to voters wanting him to restore the country from what he called the disaster created by the Biden-Harris administration policies.

He called Ms. Harris weak and lacking in any real plan to turn the country around.

“She’s the worst vice president in the history of our country,” Mr. Trump said.

Moderators pressured Mr. Trump to say whether he would sign a national ban on abortion and he answered by saying such a bill will never pass Congress and praised the Supreme Court decision to send the matter back to the states.

The moderators also asked Mr. Trump whether he would have done anything differently on Jan. 6, 2021, when a mob of his supporters rioted in the U.S. Capitol.

Mr. Trump said he told the protesters to march peacefully and that he offered 10,000 National Guard troops that day to protect the Capitol but was rebuffed by then-House Speaker Nancy Pelosi, a Democrat.

It was the second presidential debate of 2024 but the first between Ms. Harris and Mr. Trump, who met for the first time on the stage in Philadelphia’s National Constitution Center.

Mr. Trump last faced off in June against Mr. Biden, whose disastrous performance in that debate forced him out of the race a few weeks later.

Ms. Harris lost her bid to change the debate rules from the ones set when Mr. Biden faced off against Mr. Trump in June. The Harris team sought to leave the mics open, which would have given Ms. Harris a chance to challenge Mr. Trump’s answers more directly and showcase skills she honed as a prosecutor.

Mr. Trump’s campaign challenged the changes and the muted mics remained in place.

Ms. Harris was granted her request to position her traveling press pool close to the debate stage, where they were positioned to hear Mr. Trump if he speaks when his microphone is muted. Ms. Harris, who is 5-foot-4, used a shorter podium than the 6-foot-3 Mr. Trump.

Ms. Harris faced scrutiny about her true positions and pledges for a “new way forward” on the issues the same day internet sleuths determined that the issues page the Harris campaign loaded on Sunday night, which for the first time outlined her broad agenda, included metadata showing some of it was cut and pasted from Mr. Biden’s now-defunct campaign website.

The debate was framed at high-stakes for Ms. Harris, who has had just one television interview since becoming the 2024 Democratic presidential nominee.

It was an opportunity for her to define her positions more clearly for the small but significant percentage of people who are still undecided and who, polls indicate, are unsure of where Ms. Harris stands on important issues. Her complete pivots on immigration, nationalized health care, fracking, banning plastic straws, defunding the police and more has left critical voters on the fence.

Her nearly four years as vice president in the Biden administration have left Ms. Harris and her campaign struggling to separate herself from an unpopular president whose policies she enthusiastically endorsed.

Ms. Harris experienced a quick bounce after she replaced Mr. Biden on the ticket and almost immediately erased the gains in polls Mr. Trump held over Mr. Biden.

After the Aug. 19 Democratic National Convention, however, there was no additional bounce and the polls have tightened to dead even.

Ms. Harris spent several days huddled in a Pittsburgh hotel preparing for the debate while Mr. Trump followed his usual practice of campaigning and media interviews, with some informal debate preparation on the side with his surrogates, among them former Rep. Tulsi Gabbard of Hawaii, a former Democrat, and Rep. Matt Gaetz, Florida Republican.

It was the sixth presidential debate Mr. Trump has participated in as the party nominee and pressure was on the former president to hone his answers in a way that does not turn off female voters, who polls show already favor Ms. Harris.

• Susan Ferrechio reported from Jacksonville, Florida.