President Biden called in to speak with Kamala Harris' campaign HQ on Monday, saying he "isn't going anywhere" after announcing Sunday that he's stepping aside from his reelection bid.

"The name has changed at the top of the ticket, but the mission hasn't changed at all," Biden said over the phone. "And by the way, I'm not going anywhere. I'm going to be out there on the campaign with her," he added.

"I'm going to be working like hell, both as a sitting president, getting legislation passed, as well as in campaigning," Biden said.

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

Kentucky Gov. Andy Beshear said he spoke to Vice President Kamala Harris after her campaign announcement Sunday.

"The vice president reached out to me yesterday, just a couple hours after the president put out the news, and I was very grateful that she would reach out to me personally. She asked for my support. I agreed, and thus, this morning, I fully endorsed her and will do everything I can to help her win," Beshear told Louisville ABC affiliate WHAS.

Amid speculation Beshear could be a possible running mate, the governor signaled what he could bring to the campaign.

"We've shown that we know how to win. We know how to reach across the divide, and how to campaign in a way where we talk to people and not at them, where we listen to people's concerns and we give them confidence that we're going to address them," Beshear said.

"But I think it's more than just running," Beshear added. "It's also how you govern, governing in a way that addresses people's everyday needs, the things they worry about when they get up in the morning,” he said.

-ABC News' Jacob Steinberg

Kamala Harris' presidential campaign has surged in the 24 hours since announcing her candidacy, with 28,000 volunteers signing up to help organize, according to her campaign.

The boost in efforts is more than 100 times than on an average day, according to campaign officials.

-ABC News' Gabriella Abdul-Hakim

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