Obama's comments will draw a line under President Joe Biden's tenure following his stunning departure from his party's ticket last month.
"It's good to be home in Chicago," Obama posted on X ahead of his marquee appearance at the home of the Chicago Bulls basketball team to drum up excitement for Harris's campaign to deny Donald Trump a second term in the White House.
"Looking forward to being at the Democratic Convention, and joining so many inspiring people to share what's at stake in this election."
Convention delegate Ted Hiserodt, 56, said he was "very excited -- (Obama) is one of the greatest orators of my lifetime."
"He's just very good at getting the energy level high and firing up the volunteers," he told AFP.
Hiserodt and roughly 4,700 fellow delegates will anoint Harris their candidate for the November 5 poll in a ceremonial vote, repeating an online count undertaken in early August devoid of customary pomp and celebration.
One by one, members of 57 delegations will announce their picks for the party's ticket in what organizers promise "will be a celebration," including the accomplishments of the last four years.
In an emotional speech late Monday, Biden chronicled the highlights of his time in the White House, heaping praise on Harris in an appearance designed to show unity.
"It's been the honor of my lifetime to serve as your president. I love the job, but I love my country more," said the 81-year-old after a rapturous standing ovation.
"I made a lot of mistakes in my career, but I gave my best to you. For 50 years, like many of you, I gave my heart and soul to our nation," he said before embracing Harris on stage.
Biden also said it was "not true" that he was angry at people who said he should step aside -- amid reports that he was frustrated with Obama for not being more vocal in his support.
Biden was however effectively the warm-up act for Harris, who will give her keynote speech on Thursday in a slot that just a few weeks ago would have been his.
Harris paid tribute to Biden in a surprise appearance at the opening of the convention, saying she was "forever grateful" to her boss and mentor.
- 'Going to do this' -
Hillary Clinton, who lost to Trump in 2016, said Biden "brought back decency" to the White House.
"Donald Trump fell asleep at his trial. And when he woke up, he made his own kind of history. The first person to run for president with 34 felony convictions," she said to chants of "lock him up" reverberating around the arena.
The Republican former president will hold an election campaign rally on Tuesday in Michigan, a must-win swing state.
Harris will meanwhile be campaigning in Milwaukee, which hosted the Republican National Convention last month.
Harris has managed to turn the White House race on its head since Biden stepped aside, reaching out to young, female and Black voters who had switched off from a battle between two elderly men.
Trump, meanwhile, has been left reeling by what he calls a "coup" by Democrats to replace Biden.
He has struggled to recalibrate his campaign to deal with Harris, falling back instead on personal insults and rambling speeches despite appeals from top Republicans to focus.
Protesters are expected to once again take to the streets of Chicago Tuesday after a handful of pro-Palestinian demonstrators briefly breached an outer perimeter fence on Monday at the convention venue.
Despite the noisy demonstrators, morale was high among the thousands of party faithful on day one of the convention.
Franklin Delano Williams, 78, who was attending his 12th convention, said there was "no question" Obama's appearance at the convention would boost Harris's chances.
"You've seen these rallies, right? The crowds? We are turning out. We're going to do this," Williams said.
