West Virginia Rep. Shelley Moore Capito (R) announced today that she will not challenge Sen. Robert Byrd (D) in 2006, dealing a major blow to Republican efforts to gin up a competitive contest for the Senate’s longest-serving Democrat.

Capito had been aggressively courted to run by national party leaders, including White House and National Republican Senatorial Committee officials. Several publicly released polls had shown the race would have been highly competitive, and it would have become one of the marquee races of 2006 had it materialized.

But for the time being, the three-term Congresswoman said she believes she can best serve her constituents by remaining in the House. Capito is still widely expected to run for statewide office in the future, likely for an open Senate seat.

“I took a lot of time with this decision,” Capito said in a statement. “My best future right now is to ask voters to send me back to my seat in Congress. I feel I’ve been very effective there, and there’s more to be done.”

She added: “One thing I took a hard look at was the slash-and-burn politics that have developed, particularly since 2004. The negativity that would have been attached to one of the biggest races in the country is not the image of West Virginia that I want people to see.”

Byrd announced last week that he would seek a ninth term in 2006.

While Republican strategists have expressed confidence in the party’s ability to field a strong candidate against Byrd next year even if Capito decided not to run, no apparent second choice challenger has emerged.

Capito’s decision is the second high profile recruiting setback for Senate Republicans in 72 hours. North Dakota Gov. John Hoeven (R) announced Friday that he would not run against Sen. Kent Conrad (D) next year, greatly diminishing GOP hopes of a competitive contest there.