Minnesota Gov. Tim Walz made an unannounced appearance at Pennsylvania's delegation breakfast in Chicago Monday morning and spoke glowingly about President Joe Biden ahead of the president's DNC speech Monday night.

"We get an opportunity tonight to say a thank you to the best president of my lifetime that I've been able to witness, to someone who's delivered time and time again, someone who made the selfless act of handing that torch to an incredible leader," Walz said.

His speech was sandwiched between the state's Gov. Josh Shapiro and other high profile governors: JB Pritzker of Illinois and Gretchen Whitmer of Michigan.

The Harris campaign said that Walz also made a surprise appearance at the Wisconsin delegate breakfast earlier on Monday, which featured Senate Majority Leader Chuck Schumer, Wisconsin Gov. Tony Evers, Maryland Gov. Wes Moore and New York Gov. Kathy Hochul.

-ABC News' Will McDuffie and Isabella Murray

First lady Jill Biden is expected to speak Monday night at the DNC and, according to a source familiar with her remarks, will focus on the president's strength and character, as well as make clear her support for Vice President Kamala Harris.

The first lady will call on Americans to come together "with faith in each other, hope for a brighter future, and love for our country," the source said.

She will be introduced by a video highlighting key moments from her time in this role, and the issues she has focused on in her public life, according to a source familiar. The video will include Vivian Wierwille from New Hampshire, who gave a memorable introduction of Dr. Biden in March 2021 at Christa McAuliffe Elementary School during the First Lady's tour to highlight safely reopening schools after the pandemic.

While these remarks may be her last big address in her role as first lady, her office says she plant to continue to focus on her work in the next six months.

"Over the next six months, the First Lady will continue to champion the causes that have defined her public life: supporting military families, advocating for increased educational opportunities, working to end cancer as we know it, and advancing women's health research," Elizabeth Alexander, the First lady's communications director said in a statement.

The source says that the first lady will "of course" be out on the trail for Harris this fall.

-ABC News' Molly Nagle

The Democratic National Convention kicks off this morning in Chicago. According to party officials, 50,000 total visitors are expected throughout the week, including roughly 5,000 delegates and alternates and 15,000 members of the news media.

The DNC has not released a full rundown of the evening schedule but the marquee speech during convetnion programming (5:30 - 10:00 pm local time, 6:30-11:00 p.m. ET) is President Joe Biden. First lady Jill Biden and former Secretary of State – and 2016 candidate – Hillary Rodham Clinton are also set to speak tonight. Additional speakers will be announced today.

Monday's theme is "For the People." Officials say tonight's programing will highlight "the accomplishments and results President Biden delivered for people" – "with [Harris] by his side."

Delegates are also set to vote on the 2024 Democratic Party platform this evening.

-ABC News' Isabella Murray and Gabriella Abdul-Hakim