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Democrats celebrated Stephanie Herseth's (search) capture of a Republican-held House seat in South Dakota on Wednesday, their second takeaway of the year, and claimed it portends greater gains in the battle for control of Congress.

Republicans argued that local factors settled the race, but some conceded privately that a sour national mood and President Bush's slumping support contributed to the defeat of GOP contender Larry Diedrich (search).

"It's time to put partisan politics aside," said Herseth, who squeaked to a 2,981-vote triumph in an election that drew heavy investment from the two parties seeking momentum for the fall. Turnout was large, with more than 261,000 ballots cast.

Herseth, the 33-year-old daughter and granddaughter of prominent politicians, is expected to take her seat on Thursday to fill out the unexpired term of former GOP Rep. Bill Janklow. That would shave the Republican advantage in the House, and mean Democrats must win 11 more seats this fall if they are to gain control.

Democratic lawmakers were quick to trumpet Herseth's victory, three months after Rep. Ben Chandler was elected in Kentucky to a seat that had been in Republican hands.

"I don't think there's any question what the American public seeks, which is the wind of change. People are unhappy with the status quo," said Rep. Bob Matsui (search), D-Calif., chairman of the Democratic Congressional Campaign Committee.

Rep. Steny Hoyer (search), the House Democratic whip, said that "because of the energy for change, we are going to be able to put more seats in play" in November. The Maryland lawmaker argued that 35 to 40 seats are competitive at this stage, meaning that Democrats must win two-thirds if they are to mount a serious challenge to continued GOP control of the House.

Once Herseth is sworn in, Republicans will have 228 seats and Democrats 206 with one Democratic-leaning independent.

"For Democrats to have any real gains, they've got to double the playing field" of competitive seats, said Rep. Tom Reynolds of New York, who heads the Republican campaign committee.

He pointed out that the Democratic election gains had already been negated when Texas Rep. Ralph Hall switched parties to become a Republican last winter and another Texan, Rep. Jim Turner, announced plans to retire and concede his seat to the GOP.

As for Herseth's victory, Reynolds said, "There are no national implications in South Dakota. ... It's as much local issues as any race I've ever seen."

Instead, he said Herseth began the campaign better known than Diedrich after running unsuccessfully against Janklow in 2002, and was well-liked. "I can think of no other competitive seat in the country where the Democratic candidate will have universal name identification and a 30-point lead," Reynolds said.

Even so, several Republicans said private polling in South Dakota found voters in a pessimistic mood and Bush's support diminished, factors they said complicated Diedrich's task.

More than 50 percent of those polled statewide said the country was on the wrong track, and fewer than 40 percent said it was headed in the right direction, according to officials familiar with the private polling. Such questions are used to gauge the mood of the electorate, and many strategists view them as leading indicators of voting behavior.

Bush's approval was in the mid-50s as the campaign neared the end, these officials added. He carried South Dakota in 2000 with 60 percent of the vote.

A native of South Dakota who went east for college and law school, Herseth campaigned as a political centrist who understood the state's interests and could work across party lines to meet them.

Herseth and Diedrich agreed on many issues of particular concern in South Dakota, favoring legislation to require labeling of beef to show its country of origin and backing government support for ethanol, a grain-based fuel alternative.

Diedrich and the party criticized her support for abortion rights. He also sought to draw a contrast on tax cuts, saying he alone supported the permanent extension of all the reductions that Congress approved in 2001 and 2003.

Even in defeat, Republicans vowed to target Herseth's seat in the fall, when Diedrich will again be her opponent in a campaign for a full two-year term. "I'm telling the Democrats to enjoy the few months that you have it, because South Dakotans are going to send back a Republican come November," said Rep. Mike Rogers, R-Mich.

Democrats argue that national trends are working in their favor in Senate races, as well. Republican retirements in Oklahoma and Colorado have given Democrats fresh hope. Their contenders are running well in polls in Southern states such as North Carolina, South Carolina and Florida, where Republicans hope to capture open seats.

Republican concern was evident this week when the National Republican Senatorial Committee began airing television commercials criticizing Democratic challenger Tony Knowles in Alaska. Knowles supports oil drilling in the Alaska National Wildlife Refuge, but the ad says if elected, he will "join forces" with Massachusetts Sens. Edward M. Kennedy and John Kerry, the Democratic presidential candidate. Both men oppose drilling.

Knowles, a former governor, is challenging freshman Republican Sen. Lisa Murkowski, who first must overcome a primary challenge.

In the Senate, Republicans have 51 seats, the Democrats 48 with one Democratic-leaning independent.