Arizona Gov. Katie Hobbs formally endorsed Vice President Kamala Harris on Monday afternoon.

"As the last few weeks have made clear, Americans are looking for a new generation of leadership that will move past the divisiveness and unite us around our shared American values. I believe that leader is Vice President Harris, and I look forward to working with her to lower costs for Arizonans, restore reproductive freedom, and defend our democratic rights," Hobbs wrote on X.

-ABC News' Oren Oppenheim

Former Attorney General Eric Holder and his law firm Covington & Burling is expected to lead the vetting of a potential running mate for Vice President Kamala Harris, according to multiple sources briefed on the plans.

The vetting, which typically consist of requests for various documents along with formal interviews, is expected to get underway almost immediately, the sources said, given the team is operating in a truncated timeframe.

Among the names likely to be vetted are Govs. Josh Shapiro, Roy Cooper and Andy Beshear, and Sen. Mark Kelly, according to the sources.

-ABC News' Jonathan Karl and Katherine Faulders

Pennsylvania Gov. Josh Shapiro repeatedly declined on Monday to say whether he would accept an offer to be Kamala Harris' running mate, telling reporters at an event in Pittsburgh that he would "not engage in hypotheticals."

"I'm not going to engage in hypotheticals. This is a deeply personal decision the vice president will make. She will make it on her own timetable and on her own timeline," he said. "She needs to choose someone that she's prepared to govern with, campaign with, and someone that she feels most comfortable with. That decision should be made free of any sort of political pressure."

When asked whether he and Harris, who spoke Sunday, had discussed whether he is under consideration for the position, Shapiro said their conversation was "all about how we beat Donald Trump."

"I spoke to the vice president yesterday shortly after I spoke to the president. You know what we focused on? Defeating Donald Trump. Our conversation was all about how we beat Donald Trump to protect our freedoms in this commonwealth," he said.

-ABC News' Will McDuffie

The Harris campaign claims it has raised $81 million in the last 24 hours.

The haul includes donations raised across the campaign, Democratic National Committee and joint fundraising committees, it said.

"The historic outpouring of support for Vice President Harris represents exactly the kind of grassroots energy and enthusiasm that wins elections. Already, we are seeing a broad and diverse coalition come together to support our critical work of talking to the voters that will decide this election," Harris for President spokesperson Kevin Munoz said in a statement to ABC News, in part.

ABC News is not able to independently verify this information until the numbers are reported to the FEC.

-ABC News' Gabriella Abdul-Hakim, Fritz Farrow and Will McDuffie