The vetting of potential running mates for Harris has already begun, according to a source briefed on the process.

"The horse has left the barn," the source said.

The campaign does not see Aug. 7 as a deadline -- a date that marks the end of the planned virtual roll call, according to the source. The vice presidential pick is unlikely to be finalized by then.

The source said these four individuals have all been asked to submit materials: Arizona Sen. Mark Kelly; Govs. Josh Shapiro of Pennsylvania; Roy Cooper of North Carolina and Andy Beshear of Kentucky.

But that list is "not exhaustive" -- in other words, there may be another name or two, the source added.

-ABC News' Jonathan Karl

President Biden will deliver remarks about his decision to drop out of the 2024 race on Wednesday.

"Tomorrow evening at 8 PM ET, I will address the nation from the Oval Office on what lies ahead, and how I will finish the job for the American people," Biden wrote on X.

Biden announced his decision to not seek reelection in a letter addressed to his "fellow Americans" that he posted on social media on Sunday.

Harris has issued a statement on the fatal shooting of Sonya Massey, an unarmed Black woman who was shot in her own home by a deputy after calling 911 to report a possible intruder.

"Sonya Massey deserved to be safe," Harris wrote in the statement. "After she called the police for help, she was tragically killed in her own home at the hands of a responding officer sworn to protect and serve. Doug and I send strength and prayers to Sonya’s family and friends, and we join them in grieving her senseless death."

Police body camera footage was released by by the Illinois State Police Monday afternoon.

"The disturbing footage released yesterday confirms what we know from the lived experiences of so many – we have much work to do to ensure that our justice system fully lives up to its name," Harris said.

Harris said he joined Biden in calling on Congress to pass the George Floyd Justice in Policing Act -- a bill she coauthored when she served in the Senate.

"In this moment, in honor of Sonya’s memory and the memory of so many more whose names we may never know, we must come together to achieve meaningful reforms that advance the safety of all communities," Harris said.

-ABC News' Sarah Beth Hensley

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.