More than 10,000 people are expected to attend Harris' rally Tuesday night in Philadelphia, where she'll be joined by her new running mate, according to a source familiar with the plans.

That would make this the biggest event yet for the Harris campaign.

Harris is looking to build off her momentum. Pennsylvania kicks off her swing through seven battleground states in five days.

The pace of her campaign is in stark contrast to Trump's, which has only one rally scheduled this week -- in Montana on Friday.

In her first interview since her husband was named former President Donald Trump's running mate, Usha Vance sat down with Fox & Friends where she discussed her husband's "childless cat ladies" comment that has gained attention recently -- calling it a "quip."

"The reality is, JD made a quote – I mean, he made a quip, and he made a quip in service of making a point that he wanted to make that was substantive," Usha Vance said of the comments her husband made in 2021. "And I just wish sometimes that people would talk about those things and that we would spend a lot less time just sort of going through this three-word phrase or that three-word phrase."

She continued, "What he was really saying is that it can be really hard to be a parent in this country, and sometimes our policies are designed in a way that make it even harder."

She added that her husband "would never ever ever want to say something to hurt someone who was trying to have a family who really was struggling with that."

JD Vance has called the comments "sarcastic."

"Let's try to look at the real conversation that he's trying to have and engage with it and understand for those of us who do have families, for the many of us who want to have families, and for whom it's really hard," Usha Vance said on Monday. "What can we do to make it better? What can we do to make it easier to live in 2024?"

-ABC News' Kelsey Walsh, Hannah Demissie, Lalee Ibssa and Soorin Kim

Former Speaker of the House Nancy Pelosi, in an interview with Good Morning America anchor George Stephanopoulos on Monday morning, said that she spoke with President Joe Biden ahead of his withdrawal from the 2024 presidential race "asking for a campaign that would win." She also said Biden was "the only person that I spoke to" about Biden possibly withdrawing.

"The only person that I spoke to about this was the president. Other people called me about what their views were about it, and -- but I rarely even returned a call, much less initiated one," Pelosi said.

Later, she added, "I wasn't asking him to step down. I was asking for a campaign that would win, and I wasn't seeing that on the horizon."

-ABC News' Oren Openheim

Vice President Kamala Harris' choice for her running mate is the "most important" decision she has to make as her campaign gets started, former House Speaker Nancy Pelosi said.

All of her choices for a running mate appear to be good, Pelosi added.

"It's a difficult decision because they are all so great," she told ABC News' George Stephanopoulos on "Good Morning America" on Monday. "It is the most important decision for her to make. Not just about who can help win, but who can help serve and lead and whose confidence she trusts.

-ABC News' Kevin Shalvey