Spotted at the United Center snapping selfies was Michael Cohen, Trump's former attorney who was a star witness in his New York hush money trial.

The Trump campaign, meanwhile, continued to hold its daily press conference this morning just three miles away from the DNC site to "remind folks what Donald Trump is running for."

At the press conference, which focused on the issue of crime, Rep. Byron Donalds sought to counter Harris' prosecutor persona by painting her as soft on crime and pointing to some of her previous statements -- including her previous call to "redirect resources" from police.

Donalds also pushed a familiar line of attack against the Harris campaign -- that they are lacking in enough policy positions and instead are focusing on "joy and vibes."

ABC News asked the Florida Republican what type of messaging he is looking to see from Trump on the campaign trail this week amid tightening polls.

"I think this election is really a choice election, it's a contrast election, even more so than some others. We have very stark differences with Kamala," Donalds said, while pointing to the subjects of immigration, crime and foreign policy.

-ABC News' Olivia Rubin

Michigan Sen. Gary Peters appeared on stage with dozens of retired Teamsters to highlight Harris-Walz's commitment to unions.

"Kamala Harris and Tim Walz grew up just like me. And I know, I know they will fight for all of us," Peters said. "They will make it easier to raise our families and to retire with dignity. Just look at the record. As vice president, Kamala Harris helped pass legislation that finally, finally allowed Medicare to negotiate and lower the cost of prescription drugs."

Ken Stribling, president of the National United Committee to Protect Pensions, also addressed the crowd, saying, "As president, I know Kamala Harris will have our backs. She will fight for our retirement, Social Security and Medicare."

-ABC News' Brittany Shepherd

The DNC is highlighting several former supporters of former President Donald Trump this week, including everyday Americans who previously voted for him who are now backing Harris.

But Grisham, a former White House press secretary, is the first of several former Trump officials slated to speak at the convention. She described herself not only as a former supporter of Trump but as a "true believer" who has since changed her tune.

"He has no empathy, no morals and no fidelity to the truth," she said of Trump.

"When I was press secretary, I got skewered for never holding a White House briefing," Grisham added. "It's because, unlike my boss, I never wanted to stand at that podium and lie. Now, here I am behind a podium advocating for a Democrat. And that's because I love my country more than my party."

Rapper Common and gospel musician Jonathan McReynolds took to the stage.

Common changed up the lyrics to his song "Fortunate" for the DNC, rapping, "Be fortunate, y'all, for Kamala Harris."