Former President Donald Trump attempted to pour cold water on the enthusiasm at Vice President Kamala Harris' Tuesday night rally in Atlanta.

Trump claimed in a post on Truth Social Wednesday morning that the turnout was only high because artists performed ahead of her speech.

"Crazy Kamala Harris, voted the WORST Vice President in American history, needed a concert to bring people into the Atlanta arena, and they started leaving 5 minutes into her speech. I don't need concerts or entertainers, I just have to MAKE AMERICA GREAT AGAIN!!!" he said in his post.

-ABC News' Lalee Ibssa, Soo Rin Kim and Kelsey Walsh

Arizona Sen. Mark Kelly, who is seen as a strong Democratic vice presidential pick, defended the vice president's record on immigration and went on the attack against Republican vice presidential candidate JD Vance, in an interview on "Morning Joe" on Wednesday.

"Donald Trump and Senate Republicans, what did they do several months ago? We had a bipartisan bill that we negotiated faithfully with the administration, both sides of the aisle, and Donald Trump said that Senate Republicans can't vote for it," Kelly said. "He wanted to talk about this issue instead of actually fix it and JD Vance and other Republicans, they ran away from it."

He said Vance, who plans to visit the U.S.-Mexico border Wednesday, should instead be back in Washington passing legislation on the border.

"I mean, JD Vance is down here," Kelly said. "I think he's in Arizona today probably getting a photo op at the southern border. Kamala Harris is about solving problems. Donald Trump wants to take us, drag us back a decade."

Kelly said, in contrast, Harris wants to address border and immigration.

"She's worked on this problem for three-and-a-half years and it's a challenging problem. I mean, I represent a border state. But we have made progress, especially with what was put in place over the last several months," Kelly said defending Harris. "But I can't stress more that this is an issue that Donald Trump and Republicans just want to talk about."

When asked what he thought Harris needed in a running mate, Kelly said, "That's going to be something that the vice president is going to have to figure out."

"I think she's got a lot of great choices out there," he added. "My focus is not on this, and this is not about me. I mean, this is about the future of our country."

-ABC News' Fritz Farrow

Next week, Vice President Kamala Harris and her future running mate will crisscross the country together, a source familiar confirmed to ABC News.

Beginning Tuesday, the vice president and her running mate will hit all seven battleground states starting in Pennsylvania.

The source said to read too much into the fact that Philadelphia is first up, even though Pennsylvania Gov. Josh Shapiro has been out stumping for Harris and auditioning publicly for the role.

Harris and her running mate will then head to Wisconsin, Michigan, North Carolina, Georgia, Arizona and end in Nevada.

ABC News' Mary Bruce

Vice President Kamala Harris was the only candidate to qualify for the Democratic party's presidential nominating ballot, the Democratic National Committee (DNC) announced Tuesday night.

The virtual roll call to make Harris the official Democratic nominee will begin Thursday, Aug. 1, and will end on Monday, Aug. 5.

Across the country, 3,923 delegates petitioned to make Harris the Democratic nominee, according to the DNC, which noted Harris secured the support of 99% of participating delegates.