While Democratic Congressional Campaign Committee Chairman Chris Van Hollen predicted Foster's win would send out a "political shock wave," Republicans were quick to downplay its significance.
"The one thing 2008 has shown is that one election in one state does not prove a trend," National Republican Congressional Committee spokeswoman Karen Hanretty said in a statement.
Republicans had been hoping to hold on to the district that President Bush easily carried in 2004 with 55 percent of the vote. The district runs from Chicago's far western suburbs to almost the Mississippi River.
Foster defeated wealthy Republican businessman Jim Oberweis by capturing 53 percent of the nearly 100,000 votes cast in the election.
"Tonight our voices are echoing across the country and Washington will hear us loud and clear, it's time for a change," Foster told cheering supporters.
Van Hollen said Foster's win is a rebuke of the Bush administration and the GOP's apparent presidential nominee, John McCain, who helped raise money for Oberweis.
"The message to Republican candidates is that John McCain's not going to be able to save you in this election," said Van Hollen, a Maryland congressman.
Foster's special election win means he will fill the remainder of Hastert's term, which ends next January. The 66-year-old Hastert, who lost his powerful post as speaker when Democrats took control of Congress, resigned late last year.
Foster, 52, worked for 22 years at the Fermi National Accelerator Laboratory. As a young man, he and his brother started a company that manufactures theater lighting equipment.
The race between Foster and Oberweis spawned a contentious campaign that saw both men counting on high-profile supporters to sway voters. Oberweis had Hastert and McCain; Foster leaned on Democratic presidential candidate Barack Obama who endorsed him in a TV ad.
"The people of Illinois have sent an unmistakable message that they're tired of business-as-usual in Washington," Obama said Saturday night in a written statement.
Oberweis, whose name is synonymous with his family's dairy business and his financial management company, gets another shot at the seat when he squares off again with Foster in November for a new, full term in the 14th Congressional District.
"I'm really disappointed that we came up second but that's where we're at," Oberweis said.
This is the latest election disappointment for Oberweis who has lost primary races twice before for the U.S. Senate and once for Illinois governor.
With Foster headed to Washington, the district will have a rookie congressmen after years of enjoying Hastert's clout. During his two decades in Congress, Hastert funneled millions of dollars to the district. He was the longest-serving Republican speaker in history.
Hastert's is 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.
Besides attacking each other in negative TV ads, Foster and Oberweis clashed on issues ranging from immigration and health care to the Iraq war.
During a recent TV appearance, Foster said he would be a "good vote in Congress to change President Bush's policy" on Iraq. Oberweis contended the troop surge there was working.
