The president will mark Wednesday's milestone with two high-profile events, both designed to highlight the accomplishments of his fledgling administration.
Obama starts his day with a White House appearance with Sen. Arlen Specter, the veteran Pennsylvania Republican who is switching parties, a development the president said he was "thrilled" about. Vice President Joe Biden, who had long encouraged his former Senate colleague to become a Democrat, also planned to attend.
Later, Obama was to hold a town hall-style meeting in Arnold, Mo., a suburb of St. Louis, where aides say he'll note discuss his first 100 days but also focus on moving his agenda forward. Obama returns to Washington in the afternoon to prepare for a prime-time news conference, his third since taking office.
Darrell West, director of governance studies at the Brookings Institution, said the White House is smart to try to take control of the message on the 100th day.
"It's always good to play offense," West said. "If you create a vacuum, someone else is going to fill it."
Spending at least part of his 100th day on the road is keeping in step with Obama's penchant for getting out of Washington. While debate raged on Capitol Hill over his massive economic stimulus bill, Obama took his case directly to the American people, holding town hall meetings in Indiana, Florida and Illinois. When Congress passed the bill, Obama opted out of a White House signing ceremony, choosing instead to sign the bill at a Colorado science museum.
Like most of the 12 states Obama has visited in his first 100 days, Missouri is politically important. A traditional bellwether in presidential elections, Missouri went to Obama's opponent, Republican John McCain, by just a few thousand votes in 2008. Not only will Obama be eyeing Missouri in 2012, but Democrats see an opportunity to pick up another Senate seat there in 2010 when long-serving Republican Sen. Christopher Bond retires.
Obama's efforts to fix the nation's economy have drawn comparisons to President Franklin Roosevelt, who is largely responsible for the 100 days phenomenon. Roosevelt launched many of his New Deal programs during that period and, with backing from Congress, signed 15 major bills into law.
"There was a strong bipartisan effort in Congress to follow the president's lead," said David Woolner, senior vice president of the Franklin and Eleanor Roosevelt Institute.
Obama hasn't had that support. His biggest legislative accomplishment — the $787 billion economic stimulus bill — passed Congress with scant Republican backing.
Woolner said the political parties have changed since Roosevelt's day, making it difficult for presidents to achieve the same level of bipartisan support Roosevelt enjoyed. Still, he said, the first 100 days remains a relevant benchmark.
"Presidents who have been successful in this period have done a good job in setting an agenda for the coming years," Woolner said.
While Obama may lack bipartisan support, he reaches his 100th day with strong public backing. An Associated Press-Gfk poll found that 64 percent of Americans approve of Obama's 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.