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As a result, Senate Democrats will move rightward if candidates such as Mr. Webb, Bob Casey Jr. in Pennsylvania, Jon Tester in Montana and Rep. Harold E. Ford Jr. in Tennessee win their general elections.

Republicans in the chamber, meanwhile, stand to lose some of their most conservative voices. Among the incumbents in the tightest races are Sens. Rick Santorum of Pennsylvania, Conrad Burns of Montana and Jim Talent of Missouri.

Strategists from both parties expect Democrats to gain 10 seats or more in the House. But the strategists note that since pickup seats by definition come from districts held by the other party, the new Democratic lawmakers likely will be more conservative than the current Democrat caucus.