FAYETTEVILLE, N.C. — As President Biden convalesced from covid and battled the biggest political crisis of his presidency Thursday, Vice President Harris appeared before a roaring crowd of supporters in North Carolina in what amounted to an audition for the role of Democratic nominee for president.

Biden has struggled to quash Democratic doubts about his ability to beat Donald Trump ever since a stumbling performance in last month’s presidential debate, and calls for him to exit the race only grew in volume this week. New polls suggested that large percentages of Democrats think he should step aside, and Rep. Adam Schiff (Calif.) a prominent House Democrat, called for him to cede the nomination to another candidate.

North Carolina’s governor, Roy Cooper, a rare Democratic chief executive in a red state who’s sometimes mentioned as a possible running mate if Harris becomes her party’s nominee, introduced the vice president at Westover High School on Thursday afternoon. The Secret Service, which is under intense scrutiny after the failed assassination attempt against Trump, worked with local law enforcement agencies to provide heightened security at the event, prompting complaints from audience members who were promised entry and were denied.

Harris kicked off her speech by thanking U.S. service members and their families. Fayetteville, a heavily military community, is near Fort Liberty, the giant U.S. military installation formerly known as Fort Bragg.

The crowd burst into cheers as Harris took a dig at Trump’s newly announced running mate, Sen. J.D. Vance (Ohio).

“If you claim to stand for unity, you need to do more than just say the word,” she said. “You cannot claim you stand for unity if you are pushing for an agenda that deprives whole groups of Americans of freedoms, opportunities and dignity. … You cannot claim to be for unity if you try to overturn a free and fair election.”

The crowd reacted almost in unison to each remark Harris made, grumbling in agreement with every attack on the Trump-Vance ticket, and responding with emphatic cheers to pro-Biden-Harris comments.

Harris, a former senator and state attorney general from California, has toured multiple swing states this week. She made a more pronounced effort to draw on her personal background and identity at campaign events in Pennsylvania and Michigan in a bid to portray strength in the embattled Biden-Harris ticket. But the tour has also made clear that she could be a formidable nominee in her own right.