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.

Several "uncommitted" delegates say they are not planning to disrupt the speeches tonight at the DNC.

Violet Olds from Kentucky and Mohamad Khatib from Florida say they want Harris to win, but their goal is to send her a strong message.

"I'm sure we're all going to fall in line when it comes to November, but as for now, we are here to speak as Americans, supporting our Palestinian American brothers and sisters to send a message," Olds said.

Khatib said Harris is a welcome change to the top of the ticket.

"When Harris came in and became the designated nominee, she made statements that are showing her understanding and appreciation of the suffering of the Palestinian people. She made the point that she's not going to stay silent."

But Khatib said words aren't enough: "What our communities are saying is that we want Kamala Harris to take actions to support these statements that she started making."

Olds said that now, under Harris, she feels "hopeful" about the future.

"That's one of the messages that the whole of the DNC is trying to conjure: hope, joy, and that we want those abroad to have that same hope and joy that we have," Olds said.

-ABC News' Selina Wang, Tia Humphries and Mitchell Alva

The first night of the DNC is underway in Chicago, with the convention officially called to order.

Tonight's theme is "For the people" and the evening is being hosted by actor Tony Goldwyn -- who famously played a president on TV in the ABC series "Scandal."