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“Democrats are already way overextended,” Mr. Nick said. “There’s just not enough resources to protect these people. If there are even more [GOP] challengers than expected, the dams are going to break.”

Ten months is a long time, but Republicans say that even in the worst of circumstances, they are poised to pick up seats in North Dakota and Nebraska.

Democratic Sen. Kent Conrad of North Dakota is retiring, and the Democratic Senatorial Campaign Committee is fielding Heidi Heitkamp, a former state attorney general, against Republican Rep. Rick Berg. Even though Ms. Heitkamp is a respected figure, she will face serious hurdles in a state where Republicans control all 12 statewide offices and two of the three federal seats.

In Nebraska, both sides are waiting for a decision from Democratic Sen. Ben Nelson on whether he will retire. If he does, there are several strong Republican candidates, including popular Gov. Dave Heineman, who could jump in and run away with the race.

But Democrats and some political analysts say the tea party and Republican infighting could hurt the party’s chances in Nebraska and elsewhere.

Candidates in the crowded Republican Nebraska primary, including state Attorney General Jon Bruning, state Treasurer Don Stenberg, state Sen. Deb Fischer and investment adviser Pat Flynn, are scrambling to win the tea party seal of approval, but they may be alienating less-conservative November voters in the process.

Republican primary candidates in Missouri, including former state Treasurer Sarah Steelman, Rep. W. Todd Akin and business executive John Brunner, also have strong tea party credentials that Democrats say could turn off voters in a general election contest against Mrs. McCaskill.

The tea party also is providing headaches for Republican incumbents or establishment-backed figures who would have been considered safe bets in previous election cycles.

Tea party candidates also are challenging six-term Sen. Richard G. Lugar in Indiana, and Mr. Lugar warned Sunday that a primary defeat for him at the hands of state Treasurer Richard Mourdock may cost the party the Indiana seat and possibly control of the chamber as a whole.

“Republicans who are running for re-election ought to be supported by people who want to see” a Republican Senate majority, Mr. Lugar said on CNN’s “State of the Union.”

He warned of the precedents of Delaware, Nevada and Colorado in 2010, when tea party-backed candidates won primaries over establishment-backed hopefuls but then did badly in general elections that analysts had seen as good chances for Republican pickups.

“There were people who claim that they wanted somebody who was more of their tea party aspect, but in doing so they killed off the Republican chances for majority,” Mr. Lugar said. “This is one of the reasons we have a minority in the Senate right now.”

Tea party favorites also are opposing former Rep. Pete Hoekstra in Michigan for backing the bank bailouts and former Gov. Tommy G. Thompson in Wisconsin for supporting Mr. Obama’s health care bill and adding 8,500 workers to the state payroll while in office.

“There is an ongoing battle between the Republican establishment and the tea party with respect to recruiting,” said Matt Miller, a former communications strategist for the Democratic Senatorial Campaign Committee.

But Republicans say the tea party-related infighting is being overplayed in the press, considering the large number of seats in play for Democrats and the president’s vulnerability at this point in the race.

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