The attack ad put out on Sunday by a pro-Donald Trump PAC attacking Vice President Kamala Harris was made "days" before President Joe Biden announced his decision to drop out of the race, two sources familiar told ABC News.

One source describing the move as "just in case," emphasizing how they had been preparing for this.

One source with the PAC told ABC News that they are viewing the switch from Biden to Harris as a "simple pivot" because Harris "owns all of Joe Biden's failures."

The plan is to frame Harris as a "co-conspirator" in Biden's agenda -- specifically on immigration, as the team has been reporting, the source said.

-ABC News' Olivia Rubin and John Santucci

Former President Donald Trump's running mate JD Vance reacted to President Joe Biden's decision to drop out of the race, calling him "the worst president in my lifetime" and saying they are ready to take on Vice President Kamala Harris.

"Joe Biden has been the worst President in my lifetime and Kamala Harris has been right there with him every step of the way. Over the last four years she co-signed Biden's open border and green scam policies that drove up the cost of housing and groceries. She owns all of these failures, and she lied for nearly four years about Biden's mental capacity--saddling the nation with a president who can't do the job," Vance said in a statement on X.

"President Trump and I are ready to save America, whoever's at the top of the Democrat ticket. Bring it on," Vance said.

After announcing her 2024 presidential campaign, Kamala Harris has put out a public call for fundraising.

"I will do everything in my power to unite the Democratic Party -- and unite our nation -- to defeat Donald Trump and his extreme Project 2025 agenda. If you’re with me, add a donation right now," Harris wrote on X Sunday.

The statement includes a photo with President Joe Biden, who endorsed Harris immediately following his exit from the race.

There are practical concerns that could dissuade other Democrats from challenging Vice President Kamala Harris's campaign for president, former New Jersey Gov. Chris Christie, an ABC News contributor, told ABC News Live on Sunday.

"There are such practical concerns for the people who might be thinking about challenging the vice president. None of them have national campaigns organized, none of them have one state director in any one of the 50 states, none of them have a national fundraising operation," Christie said.

"I think she'll be able to inherit the money that is in the Biden-Harris campaign right now," Christie said.

Christie added that anyone considering running will wonder if they will be able to beat a candidate who is a sitting vice president who has been endorsed by the sitting president.

"Most of the people who would be serious candidates, I think, quite frankly, they are going to look at this and say 'I hope she picks me for No. 2' and move on from there," Christie said.

"I don't think there is going to be any challenge to Kamala Harris," Christie said.