While the vetting process is just getting started, a senior administration official told ABC News' Martha Raddatz that Pennsylvania Gov. Josh Shapiro and Arizona Sen. Mark Kelly are the leading candidates to be Harris' running mate.

The two men come from key 2024 battleground states, and have experience trouncing Trump-endorsed candidates.

Shapiro, 51, was Pennsylvania's attorney general before being elected governor in 2022 after defeating Republican Doug Mastriano.

Kelly, a 60-year-old former astronaut and Navy captain who is married to former congresswoman Gabby Giffords, won his Senate seat in 2022. He defeated Republican challenger Blake Masters.

Actor and Democratic megadonor George Clooney, whose op-ed in the New York Times calling on Biden to step aside ignited a political firestorm, told CNN he is committed to supporting Vice President Harris.

"President Biden has shown what true leadership is. He's saving democracy once again. We're all so excited to do whatever we can to support Vice President Harris in her historic quest," he said in a statement first provided to the network.

Vice President Kamala Harris will on Tuesday hold her first presidential campaign rally in Milwaukee, Wisconsin, according to her campaign.

The Harris campaign signaled in a memo that the vice president would take on former President Donald Trump and his running mate, Sen. JD Vance, on abortion and Project 2025.

The campaign said Harris "is prepared to prosecute the case against Donald Trump -- and there’s no better place to drive this contrast than Milwaukee, Wisconsin."

The rally comes days after Republicans held their nominating convention in Milwaukee, and weeks before the Democrats hold their convention in Chicago.

"Vice President Harris’s visit will highlight the choice facing Wisconsinites: between Donald Trump, the convicted felon who would drag this country backwards, and her brighter vision for the future, where our freedoms are protected and every American has a fair shot," the campaign said in the memo.

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

Vice President Kamala Harris has raised $100 million since entering the race for president on Sunday, according to her campaign.

The massive haul was raised between Sunday afternoon --when President Joe Biden announced he would step down from the ticket—and Monday evening, the Harris campaign said. The Democratic National Committee and its joint fundraising committees raised the money, the campaign said.

About 58,000 people also signed up to volunteer for the vice president's campaign, a figure that is more than 100 times its average daily signup rate, according to the campaign.

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