After a warm welcome from the crowd, former House Speaker Nancy Pelosi began her remarks thanking Biden for his "patriotic vision of a fairer America."

"Millions of jobs. Stronger infrastructure. A Biden child tax credit. Rescuing union pensions. Honoring our veterans. Bold climate action. Lowering the costs of prescription drugs," she said of the presidency's accomplishments, calling it one of the most successful of modern times.

Pelosi said Harris is now "ready to take us to new heights."

"Officially, she is a leader of strength, with wisdom and eloquence on policy -- most recently demonstrated fighting for women’s right to choose. Politically, she is astute and strategic in winning difficult elections -- quickly securing the nomination with dignity and grace and choosing Tim Walz as our vice president," Pelosi said.

Former House Speaker Nancy Pelosi is addressing the DNC after appearing to play a significant role in Biden's exit from the top of the Democratic ticket.

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Nancy Pelosi takes the stage at DNC

After a warm welcome from the crowd, former House Speaker Nancy Pelosi began her remarks thanking Biden for his "patriotic vision of a fairer America."

Pelosi, who last month endorsed Harris to be the party's nominee for president, did not publicly call on Biden to withdraw from the race. Instead, Pelosi, in a July 10 interview, declined to directly answer when asked if Biden had her support in his reelection bid after his ruinous debate performance.

At the time, Biden had already committed to running for reelection, writing in a statement congressional Democrats days earlier that he was "firmly committed" to staying in the race.

Her comments blunted any progress Biden was trying to make persuading congressional Democrats that he was up for the job. And Pelosi's ambiguous public comments instead created space for rank-and-file Democrats to pressure the president to drop out.

-ABC News' Sarah Beth Hensley, John Parkinson, Lauren Peller and Allison Pecorin

Bill Clinton, like many of this week's keynote speakers, is advising Democrats at the United Center and around the country not to get distracted or overconfident this election cycle.

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Bill Clinton says 'clear choice' in election during full DNC speech

"After the last two days, aren't you proud to be a Democrat?" he asked the crowd as he began his remarks.

Seemingly referencing the 2016 election, he said: "We've seen more than one election slip away from us when we thought it couldn't happen. When people got distracted by phony issues.

"This is a brutal, tough business. I want you to be happy," he continued.

"One of the reasons that Vice President Harris is doing so well is that we're all so happy. But you should never underestimate your adversary."

Speakers at the DNC have mentioned the word "joy" more than two dozen times tonight.

The most recent reference came from Bill Clinton.

"Like Hakeem Jeffries, I too want an America that's more joyful, more inclusive, more future focused," he said.

"Just think what a burden that's been on us to get up day after day after day after day, buried in meaningless hot rhetoric, when there's so many opportunities out there, so many problems that need to be solved."