March 9 (Bloomberg) -- Democrat Bill Foster won a special election for the Illinois U.S. House seat that former Republican Speaker of the House Dennis Hastert quit last year, giving Democrats an early, upset victory in their election-year bid to expand their majority.

Foster, a scientist and business owner, had 52.5 percent of the vote to Republican Jim Oberweis's 47.5 percent and was declared the winner by the Associated Press based on unofficial results tabulated by the wire service.

Analysts said the election is a sign that Republican losses are likely to continue this year, after the party lost control of the House and Senate in 2006.

``I don't think it means the end of the Republican party like some might say, but it means the worst is not over,'' said Nathan Gonzales, political editor of the Rothenberg Political Report in Washington.

Democrats said the result was a ``shock wave'' in advance of elections in November that will decide control of the White House, the House of Representatives and Senate.

``Bill Foster's victory in the seat that was held by Speaker Hastert sends a political shock wave across America this election year,'' said Representative Chris Van Hollen of Maryland, the chairman of the Democratic Congressional Campaign Committee. ``It is a stunning rejection of the Bush administration, its Republican allies, and presidential nominee John McCain.''

`No National Trend'

Republican party officials rejected the idea that the race is a bellwether for other contests later this year.

``The one thing 2008 has shown is that one election in one state does not prove a trend,'' said Karen Hanretty, communications director for the National Republican Congressional Committee. ``In fact, there has been no national trend this entire election season.''

The victory, in a district held by Republicans for more than 30 years, boosts Democratic hopes that the party is poised to win more seats in the November elections and gain firmer control of the U.S. House.

The vote also may demonstrate the ability of presidential contender Barack Obama, an Illinois senator, to aid congressional candidates. Obama appeared in a campaign commercial for Foster, calling him another Illinois Democrat who can ``deliver change.''

Foster, 52, a former physicist at the U.S. Energy Department's Fermilab National Accelerator Laboratory outside Chicago, also was endorsed by the Chicago Tribune.

Presumptive Republican presidential nominee John McCain held a fundraiser for Oberweis, a dairy owner, last month in Aurora, Illinois.

Increasingly Competitive

The vote may signal that the district, which stretches from the Chicago suburbs almost to the Mississippi River, became increasingly competitive during Hastert's almost 11 terms in office. The 60 percent of the vote that Hastert received in 2006 was his lowest winning margin since 1986.

President George W. Bush carried the district with 55 percent of the vote in 2004.

Hastert, 66, the longest-serving Republican House speaker in history, left his leadership post in the wake of the 2006 elections. In November he resigned his seat in Congress.

Foster will complete the two-year term Hastert was elected to in November 2006. The seat will again be on the ballot this November.

Republican Debts

The former speaker's decision to resign rather than serve the remainder of his term was costly for House Republicans, who are deeply in debt after 2006 elections that cost them their majority. The National Republican Congressional Committee spent about $1.2 million defending the seat, according to reports filed with the Federal Election Committee. The rival Democratic Congressional Campaign Committee spent about $1 million working to gain the district, FEC reports show.

Republicans face a challenging election year in Congress. So far, 29 House Republicans have resigned, announced they won't seek re-election or lost a primary race, with close to half of those seats rated competitive by the nonpartisan Cook Political Report. Democrats are defending just seven open seats so far this year.

With the addition of Foster, Democrats have 232 seats in the House, while Republicans have 198. There are five vacancies.

To contact the reporter on this story: Laura Litvan in Washington at llitvan@bloomberg.net;

Last Updated: March 9, 2008 09:37 EDT