By Greg Giroux, CQ Staff

Illinois Democrat Bill Foster defeated Republican Jim Oberweis in a special election Saturday in Illinois’ 14th U.S. House District — a stunning defeat and embarrassment to the Republican Party in a race that carried outsized symbolism because it had been held by former GOP Speaker J. Dennis Hastert.

Foster, a scientist and a first-time candidate, led Oberweis by 52 percent to 48 percent in nearly complete returns after a contentious and expensive race in which the national political party organizations were active participants and both candidates spent heavily from their deep pockets.

“Bill Foster’s victory in the seat that was held by Speaker Hastert sends a political shock wave across America this election year,” Maryland Rep. Chris Van Hollen , the chairman of the Democratic Congressional Campaign Committee, said in a statement.

Karen Hanretty, the communications director for the National Republican Congressional Committee, issued a statement that said “one election in one state does not prove a trend.”

“The one message coming out of 2008 so far is that what happens today is not a bellwether of what happens this fall,” Hanretty said.

Foster’s win marked the first time since 2004 that a political party won control of a U.S. House district from the opposite party in a special election. Upon Foster’s swearing in, he will become the 232nd Democrat in the U.S. House, which has 198 Republicans and five vacancies — three in districts where the Republicans are the defending party.

Foster will represent Illinois’ 14th, which includes suburbs west of Chicago and rural territory reaching nearly to the Mississippi River, through the end of this year. He and Oberweis also will compete in November to determine who will serve a two-year term that begins next January.

At the time of Hastert’s resignation last November, Foster was an underdog in the race to succeed him — in part because Illinois’ 14th is a Republican-leaning district in which President Bush won 55 percent of the vote in the 2004 election.

But Bush is now much less popular, and Foster ran a campaign that Democrats said was a referendum on the Bush administration, including its economic policy and its prosecution of the war in Iraq. Foster was endorsed in a television ad by Illinois Sen. Barack Obama , the Democratic presidential front-runner whose calls for “change” in Washington were echoed by Foster.

Oberweis, whom Hastert backed over state Sen. Chris Lauzen in the Feb. 5 primary election, was waging his fourth bid for political office in as many election cycles — he lost Republican primaries for U.S. senator in 2002 and 2004 and for governor in 2006. Oberweis tried to paint Foster as an “extreme” supporter of higher taxes.

Early returns on Saturday night augured well for Foster, who led Oberweis in Kane County, which includes Aurora and Elgin and is the most populous of the eight counties that are wholly or partly within the 14th’s boundaries. Kane had given Bush 55 percent of the vote in 2004 and also backed Republican Judy Baar Topinka in her losing 2006 campaign for governor against Democratic incumbent Rod R. Blagojevich .

Foster even edged Oberweis in Kendall County, a Republican bastion that is Hastert’s political base and that gave 61 percent of the vote to Bush in 2004. Foster won decisively in DeKalb County, which includes Northern Illinois University, and was slightly ahead in DuPage County, the part of the 14th that is closest to Chicago.