(AP Photo/Gerald Herbert)
"I'm confident in the future but I'm not content with the present," the president told a town-hall style event in a St. Louis suburb.
Later, the president planned to head back to Washington to send that same message to the rest of the country at a prime time news conference.
Even as his administration sought to minimize the symbolism of the 100-day marker, the White House staged these two high-profile, high-visibility events to promote Obama's accomplishments while pressing his big-ticket agenda.
In office just three months, the Democrat enters the next phase of his new presidency with a high job approval rating and a certain amount of political capital from his history-making election last fall. But he also faces a thicket of challenges as he seeks to move ahead on multiple fronts both foreign and domestic amid recession and war. He will need continued public support to accomplish his lofty goals.
Thus, Obama used the anniversary -- some aides derided it as a "Hallmark holiday" -- to travel to Missouri to press his case.
"We have begun to pick ourselves up and dust ourselves off, and we've begun the work of remaking America," Obama proclaimed. But he acknowledged, "We've got a lot of work to do because on our first day in office, we found challenges of unprecedented size and scope."
He defended his ambitious, costly plan, saying: "These challenges could not be met with half measures. They couldn't be met with the same, old formulas. They couldn't be confronted in isolation. They demanded action that was bold and sustained."
And, Obama countered critics who said he's taking on too much, as he works to turn around the recession while revamping energy, education and health care in the United States.
"The changes that we've made are the changes we promised," Obama said. "We're doing what we said we'd do."
Earlier, Obama began his day at the White House, where he welcomed Sen. Arlen Specter, the veteran Pennsylvania Republican, to the Democratic Party. The president said he was "grateful" for Specter's decision to switch parties. Vice President Joe Biden, who had long encouraged his former Senate colleague to become a Democrat, also attended.
The president then darted to Missouri to hold what aides billed as a question-and-answer event, though Obama spent roughly 20 minutes making opening remarks in which he touted changes his fledgling administration already has made and other issues it wants to tackle in the coming months.
Obama drew a standing ovation from the crowd as he noted his first 100 days, saying: "That's a good thing." He also hailed the day as "the beginning of another long journey," given the challenges facing the country. And he said he's pleased the public seems to understand that.
The president promised to fight for everyday Americans, saying: "My campaign was possible because the American people wanted change. I ran for president because I wanted to carry those voices, your voices, with me to Washington. So I just want everybody to understand you're who I'm working for every single day in the White House. I've heard your stories. I know you sent me to Washington because you believed in the promise of a better day. And I don't want to let you down."
Obama reached his 100th day with strong public backing. An Associated Press-GfK poll found that 64 percent of Americans approve of his job performance and 48 percent believe the country is headed in the right direction. The "right direction" number is up 8 points since February and 31 points since October, the month before Obama's election.
But problems may lurk behind that public optimism. Ninety percent of Americans consider the economy an important issue -- the most ever in an AP poll -- and 65 percent said it's difficult for them and their families to get ahead.
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