Braley coasted to victory in eastern Iowa, beating first-time Republican candidate Mike Whalen in a race targeted by both parties as critical to their hopes in the 2006 midterm elections.
Braley, who ran a campaign criticizing Republicans and President Bush on health care and the Iraq war, outpaced Whalen 55-43 percent with 89 percent of the precincts reporting.
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His victory was part of a Democratic surge in the state.
Incumbent Democrat Leonard Boswell was elected to a sixth term in Congress, giving Iowa two Democrats in the House for the first time since 1992. Democrats also captured the governorship and had a chance to claim both chambers of the Legislature.
Boswell beat Republican Jeff Lamberti, the state Senate co-president, in a campaign that was costly and bitter to the end.
"It's been a pretty hard few days, but you have won," Boswell told supporters in his victory speech. "We've been through some rough times in our lives and we think it's time for America ... to say 'Let's get a handle on the way campaigns are run and let's get back to the truth."'
As expected, Republicans held on to two other House seats.
Rep. Steve King beat Democrat Joyce Schulte in the 5th District. In the 4th District, Rep. Tom Latham defeated Democrat Seldon Spencer, a neurologist from Ames.
In Iowa's 2nd Congressional District, Republican incumbent Jim Leach was locked in a tight race with Democrat Dave Loebsack, a Mount Vernon college professor.
With about 178,000 votes counted, Loebsack was leading 51 percent to 49 percent.
Leach, a moderate Republican who bucked party orthodoxy by voting against the Iraq war, is seeking his 16th term in Congress.
The race may also be complicated by technical problems in Johnson County, a key Democratic stronghold. Johnson County Auditor Tom Slockett said technical problems will delay counting about 40 percent of the county's votes.
Slockett said that the procedure used for counting absentee ballots was malfunctioning, and that many ballots were not being counted. The only solution to the problem, Slockett said, was to rescan the ballots.