By Teresa Carson

PORTLAND, Ore (Reuters) - A former Oregon state lawmaker handily won a special election on Tuesday to fill the congressional seat vacated in August by disgraced fellow Democrat David Wu, who resigned in a sex scandal, state returns showed.

Suzanne Bonamici, a former Federal Trade Commission lawyer who most recently served as a state senator, garnered 54 percent of the vote to defeat Republican businessman Rob Cornilles, who ended with 39 percent, a website for the state elections office showed.

Two minor candidates split the remainder of the vote, which like all Oregon elections was conducted by mail.

Cornilles conceded defeat in a speech to supporters in which he gave no indication of an intent to run again when Bonamici will need to seek re-election in November.

He has now lost two bids for the same seat, after unsuccessfully challenging Wu in 2010 in an election year that heavily favored Republicans elsewhere in the country.

Paul Gronke, a political analyst and professor at Reed College in Portland called Tuesday's outcome a decisive win for Bonamici in a western Oregon district that has sent only Democrats to the U.S. House of Representatives since 1974.

"There is no sign here of any erosion in what is Democratic territory," he said.

The district includes part of Portland, Nike Inc.'s headquarters, much of Oregon's high-tech industry, affluent suburbs and areas dependent on farming and fishing.

The special election attracted an unusual level of interest for a traditionally safe Democratic seat, drawing almost $2 million in independent expenditures since November.

Democrats hold a 12-percentage-point edge over Republicans in voter registration in the district. President Barack Obama easily carried the district in 2008 and early polls had showed Bonamici with a considerable lead.

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Still, the Democratic Congressional Campaign Committee spent about $1.3 million on the race, much of it for television and Internet messages painting Cornilles as a conservative extremist by recalling that he referred to himself in his race against Wu as "the original Tea Party candidate."

Ads from the Cornilles camp, meanwhile, portrayed Bonamici as a proponent of higher taxes and showed photos of her with Wu, a seven-term incumbent who resigned after he was accused of an unwanted sexual encounter with a campaign donor's 18-year-old daughter.

Democrats' strategy of attacking Republicans by casting them as having curried favor with the Tea Party was expected to play out many times more in their quest this year to retake congressional seats lost in 2010.

Polls have suggested declining voter sympathy for Tea Party-sanctioned Republican freshmen in the House of Representatives after they were seen as obstructionist on the debt ceiling, extending payroll taxes and other issues in 2011.

On the lighter side of the campaign, Cornilles recently produced a satirical video of himself as a Stephen Colbert look-alike character named "Robert Colbert" (pronounced Ro-BARE Col-BARE), who introduces himself as "the estranged younger brother" of the political humorist and faux conservative TV commentator.

The 3-minute clip, in which Cornilles mimics the comedian while poking fun at his resemblance to both Colbert and actor Matthew Broderick, was posted over the weekend on YouTube and picked up on the political news website, Politico.com.

"Following an interesting but unusual stint as a Matthew Broderick stand-in, I decided to lower my expectations. What about Congress?" he deadpans into the camera at one point.

(Additional reporting and writing by Steve Gorman; Editing by Cynthia Johnston and Peter Bohan)