A day after snagging former Republican House Speaker Dennis Hastert’s congressional seat in a special election, Democrat Bill Foster knew Sunday he didn’t have much time to savor the victory.

Foster’s weekend win to fill the remainder of Hastert’s term until January will be a fleeting one for Democrats unless he can hold on to the seat in the fall election for a new, full term.

“The best way to establish a long-term career in Congress is to do a good job from Day One,” Foster told The Associated Press in a telephone interview while en route to a restaurant to thank voters.

A millionaire physicist and businessman, Foster defeated wealthy Republican Jim Oberweis after a contentious campaign with 53 percent of the nearly 100,000 votes cast in the special election Saturday.

The longest-serving Republican speaker in history, Hastert resigned the seat late last year after losing his powerful speaker post when Democrats took control of Congress in the 2006 election. He spent two decades in Washington.

Foster chalked up his win to voters dissatisfied with Republicans and to demographic changes caused by ever-spreading suburban sprawl. The district stretches from Chicago’s far western suburbs to almost the Mississippi River.

Foster credited high-profile support from some well-known Illinois Democrats. Presidential candidate Barack Obama endorsed him in a TV ad.

Foster said he heard someone at a victory party say “Barack has pretty broad coattails for someone who’s so skinny.”

Democratic Congressional Campaign Committee Chairman Chris Van Hollen said Foster’s win sets him up for the November election _ again, against Oberweis _ because he can build on his momentum.

Foster won a Democratic primary by less than 400 votes to be the candidate in the November race, although one challenger has initiated a re-count.

Republicans have downplayed Foster’s win, with National Republican Congressional Committee spokeswoman Karen Hanretty saying in a statement that “one election in one state does not prove a trend.”

Hastert’s was one of three open seats in Illinois this year because of GOP retirements. Reps. Jerry Weller and Ray LaHood are also stepping down. The Democratic Party’s chances to pick up one of those seats improved when the Republican nominee to replace Weller dropped out of the race.

But right now, Foster, 52, is focused on bringing his past work as a physicist and businessman to bear on his new job as a rookie on Capitol Hill.

“The scientific training teaches you always to look at the facts first. If you look at the places this country’s gotten itself in trouble, it’s very often where we ignore facts for political reasons,” Foster said, citing the Iraq war.

Foster worked for 22 years at the Fermi National Accelerator Laboratory. As a young man, he and his younger brother started a company that manufactures theater lighting equipment. He said he was bought out of the company after deciding to get into politics so he could avoid any conflicts.

He said his win carries a message for both political parties. “People on both sides should take the message that we want people who are less divisive, less ideological and more problem-solving,” he said.