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Major pollsters have not followed the race, so it remains unclear which Democrat holds the edge. Each campaign has released internal polling putting its candidate ahead.

Oliverio campaign manager Curtis Wilkerson said his candidate is “vehemently” opposed to the cap-and-trade energy legislation that Mrs. Pelosi helped push through the House in the summer of 2009. The legislation is considered a job killer for the state’s coal-mining industry, and Mr. Mollohan voted against it.

“But this is not a race about Nancy Pelosi,” he said. “This is a race about the corruption of Alan Mollohan.”

Larry Sabato, director of the University of Virginia’s Center for Politics, said the strategy against Mrs. Pelosi is unusual because few conservative Democrats like Mr. Oliverio are challenging incumbents.

“I’ve looked all over,” he said. “Mr. Oliverio is a very rare exception.”

Pam Van Horn, campaign manager for Mr. Mollohan, said the congressman has always focused on the best interests of his district, including bringing high-tech jobs to the region. She said the Justice Department investigation into Mr. Mollohan was politically motivated and urged voters to “move on.”

Despite Mr. Mollohan’s long tenure, Mr. Berman said, the incumbent might be an easier target for West Virginia Republicans than the lesser-known Mr. Oliverio.

“He’s a devil we know,” Mr. Berman said. “But I feel that with either candidate we still will not have difficulty in making the case it’s time for a Republican.”

After a relatively slow week on the political scene, both parties are gearing up for major clashes May 18.

Sens. Blanche Lincoln of Arkansas and Arlen Specter of Pennsylvania face strong Democratic primary challenges. In Kentucky, both parties will hold spirited races for the open seat being vacated by Republican Sen. Jim Bunning.