What to Know About Kamala Harris, Joe Biden’s V.P. Choice

Part of her influence was ideological: She was the first candidate, for instance, to suggest requiring federal preclearance for state abortion restrictions, a position that most of the Democratic candidates later expressed support for.

She had an electric moment in the first debate last June when she forcefully challenged Mr. Biden over his record on race. The way that exchange began was also notable: The moderators had not called on Ms. Harris, but she asserted herself by saying, “As the only Black person on this stage, I would like to speak on the issue of race.”

The interaction was an example of a quality that Ms. Harris’s campaign worked hard to play up, just as it played up her intense questioning in Senate hearings. Her team’s argument was that she could take control of any debate stage, including against President Trump, and that she would not be intimidated by Mr. Trump’s bullying style of politics.

Broadly, however, she seemed to lack a guiding political ideology and sometimes struggled to present a cohesive platform, most prominently on health care. In one well-publicized instance, she vacillated on whether enacting “Medicare for all” would mean eliminating private insurance.

To many political observers, it might have seemed that the relationship between Ms. Harris and Mr. Biden was fraught. After all, in that first debate, Ms. Harris unleashed perhaps the most forceful — and memorable — attack on a rival of the entire primary campaign when she challenged Mr. Biden over his past opposition to busing as a means of integrating public schools.

It was “hurtful,” she said, to hear Mr. Biden speak positively about working with segregationist senators, because “there was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day, and that little girl was me.”

Among some of Mr. Biden’s advisers, her debate-stage attack generated lingering resentment — even as recently as weeks ago. At a fund-raiser in March, Jill Biden, the former second lady, called Ms. Harris’s debate stage remarks a “punch to the gut.”