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Making matters worse for Democrats, seven-term Republican Rep. Shelley Moore Capito, the daughter of a former governor, announced in November that she would challenge Mr. Rockefeller for his seat.

“The West Virginia seat is a super-prime opportunity for Republicans in 2014,” said Larry J. Sabato, the head of the University of Virginia’s Center for Politics. “The state has essentially turned red, and the GOP has a superb candidate in Shelley Moore Capito, who has her father’s political genes.”

Mr. Crichlow said it would have been a tight race between Mrs. Capito and Mr. Rockefeller, and said that Mrs. Capito has to be “considered the favorite” because it is not clear who Democrats will put up against her.

“You have these two giant names in Democratic politics [in West Virginia] — one being Rockefeller and one being Manchin — and one of them is gone forever,” Mr. Crichlow said, alluding to the family of the state’s other senator, Joe Manchin III. “That is a big thing. It takes a long time in West Virginia to develop a name for yourself. So, when you lose someone at that level, it is going to be a blow to the party.”