Former President Clinton's address at the DNC will be hopeful and aspirational, according to a source familiar with its contents.

The source also said it will include fiery, newsworthy jabs aimed at former President Donald Trump and will highlight the qualities that make for a responsible, qualified commander in chief in the nation's highest office.

The speech is expected to highlight the striking differences in vision, experience and temperament between Harris and Trump, underscoring the vice president's story and what her candidacy means for the nation.

-ABC News' Gabrielle Abdul-Hakim

The third night of the DNC is officially underway, with New Jersey Sen. Cory Booker calling the convention to order.

The theme is "A fight for our freedoms" and will feature speeches from former President Bill Clinton and Walz, as he accepts the party's nomination for vice president.

Conventions are also about parties identifying those they view as future leaders.

Maryland Gov. Wes Moore's coveted speaking spot tonight will not go unnoticed.

His team said this night is about the future, forward momentum, and he is grateful to be a part of that.

He knows Democrats need to show they are delivering actual results, and he thinks he has a story to tell there. A senior staffer to the governor told ABC News he will focus on the Key Bridge collapse and talk about what Biden and Harris did to support his state.

Moore also has a personal relationship with Walz, who reached out to him when he first won and has acted as a sounding board for the governor.

-ABC News' MaryAlice Parks

As I reported on Monday, political conventions in Chicago have a long and storied history. And some of that history can still be found today.

At the 1920 Republican National Convention, the GOP couldn't decide on a presidential candidate to save their life. After four rounds of balloting on June 11, no candidate had come close to clinching a majority of delegates. That night, a group of party leaders convened in Room 915 of the Blackstone Hotel and decided that Ohio Sen. Warren G. Harding would be an acceptable compromise choice. The following day, after a few more deadlocked votes, delegates decided to follow their lead and switched their votes to Harding, who won the nomination on the 10th ballot.

Those party leaders didn't technically have the power to nominate Harding themselves, but a popular image soon emerged of Republican elites secretly pulling the strings to get their preferred candidate elected. As an AP reporter put it (probably exaggeratedly), "Harding of Ohio was chosen by a group of men in a smoke-filled room early today as Republican candidate for president." The phrase "smoke-filled room" captured the public's imagination and quickly became shorthand for when party leaders coordinate behind the scenes to anoint a candidate.

History buffs will be pleased to learn that the "smoke-filled room" still exists today! Although the room (along with the whole hotel) has been completely renovated, the original fireplace is still there, and the room is decorated with political paraphernalia. If you have the coin, you can even stay there yourself! When I visited on Monday, the room was vacant, but it was booked for Tuesday through Saturday. But it isn't a political bigwig who's staying there this week — it's apparently a musician!

—538's Nathaniel Rakich