DNC Chair Jamie Harrison took the stage to tout the Democrats' work over the last four years and the goals of a Harris-Walz administration.

"The fight continues, and there is still more work to do," he said.

"I know that when we elect Kamala Harris and Tim Walz, they will build on our accomplishments and they will create a country that works for every American," he added.

Harrison encouraged voters to be active in the coming weeks for all races down the ticket.

"In my beloved home state of South Carolina, our motto is, 'While I breathe, I hope.' Folks, I believe in the power of hope, but it's going to take more than hope to win this election," he said.

Delegates and other attendees here in the United Center are amped up -- and have been paying attention.

Chicago Mayor Brandon Johnson got a rousing applause when he said his daughter will be able to see herself in Harris. And when Democratic National Committee Chair Jamie Harrison started off with one of Harris' marquee remarks, the crowd quickly finished it with him.

"When we fight…" Harrison said.

"We win!" the crowd erupted.

-ABC News' Tal Axelrod

Three vulnerable Senate Democrats will sit out the DNC this week: Sens. Jacky Rosen, D-Nev., Jon Tester, D-Mont., and Sherrod Brown, D-Ohio.

A spokesperson from Rosen's campaign said she's choosing to focus on her own reelection this week and will meet with Nevada voters instead. She did, however, join Harris and Walz at a campaign event in Las Vegas earlier this month.

"Senator Rosen supports the Harris-Walz ticket and was glad to join them on the campaign trail in Las Vegas this past weekend," a spokesperson for her campaign said.

A spokesperson for the Tester campaign similarly confirmed he won't be in attendance this week.
Notably, he'll be attending a Pearl Jam concert in Montana this week, with an associated fundraiser for his campaign with the band's bassist the day before. He'll spend the rest of his August farming and meeting with voters face to face, per his campaign.

A spokesperson for Brown also confirmed he won't be in attendance at the convention. A campaign spokesperson did not respond to multiple emails asking for further information about where the senator will be this week.

-ABC News' Allison Pecorin

Chicago Mayor Brandon Johnson helped kick off the first night of the DNC, welcoming the delegates to the city.

He told the crowd that Chicago is where former President Barack Obama "rose to the White House, reminding us that, yes, we can."

"This city of hard work and caring people is where Democrats will celebrate President Joe Biden and nominate Kamala Harris for president of the United States of America," he said.

Johnson spoke about what a Harris presidency would mean for his family.

"As a Black man raising a little Black girl on the West Side of Chicago, I know that my daughter, Braedyn, will see not only a reflection of herself in the White House, but she will experience the deepest part of American values," he said to applause.