For Republican strategists in Delaware and Washington, D.C., Rep. Mike Castle’s Oct. 6 announcement that he will run for Senate was a double-edged sword that produced both favorable and challenging consequences for the party.

While Castle’s Senate candidacy gave them a top-flight candidate for the effort to win a Democratic-held seat, it also created an opening in his at-large House seat that will be very difficult for Republicans to defend.

The Delaware race also poses a test of the GOP’s ability to compete in the Northeast, an area where there was once a vibrant centrist wing of the Republican Party ­— exemplified by Castle — but which has trended sharply to Democrats.

With the Democratic establishment firmly behind former Lt. Gov. John Carney (D), who began campaigning for the House seat six months ago, Delaware Republicans are beginning to vet potential candidates and hope to settle on a standard-bearer promptly.

The race is an unenviable task for any Republican in a state that has been drifting more and more Democratic. Although President Barack Obama won the state with 62 percent in 2008 — a total that was no doubt inflated by the presence of then-Sen. Joseph Biden (D-Del.) on the ballot as Obama’s running mate — other demographic and political trends have contributed to an unmistakable trend away from the GOP in Delaware.

In the past five presidential elections, the Democratic nominee has each time prevailed in the First State by at least 7 points. Democrats control both chambers of the state Legislature, and Castle and state Auditor Thomas Wagner are the only statewide elected Republican officials.

“I think we’re expecting John Carney to do very well in that race,— said Joseph Pika, a political scientist at the University of Delaware. “I think any Republican who would challenge him is going to have a really stiff challenge.—

Republican officials expect to field a single candidate for the race, given that Delaware’s Sept. 14 primary is one of the latest on the 2010 election calendar and is just seven weeks before the Nov. 2 general election. As a general rule, political parties try to avoid multicandidate primaries late in election cycles, given the intraparty wounds that develop and take time to heal and the likelihood that the eventual nominee will be at a financial disadvantage in a short general election campaign.

Numerous Republicans in Delaware have floated their names as potential candidates in the immediate aftermath of Castle’s announcement.

One Republican frequently mentioned as a potential candidate is Charlie Copeland, a former state Senator who won 39 percent of the vote last year as the nominee for lieutenant governor.

Priscilla Rakestraw, the Republican national committeewoman, noted that Copeland, who couldn’t be reached for comment, owns a business and is an experienced statewide campaigner.

Also frequently mentioned as possible candidates are state Reps. Tom Kovach and Greg Lavelle, as well as Robert Harra, the president and chief operating officer of Wilmington Trust, and Anthony Wedo, a restaurant and investment executive.

Also mentioned, but seen as unlikely to run, are Colm Connolly, a former U.S Attorney for Delaware who received substantial press attention for his high-profile prosecutions, and Alan Levin, a drug store executive who serves in the cabinet of Gov. Jack Markell (D) as his director of the Delaware Economic Development Office.

That Republicans have a wide-open field of potential candidates is something of a novelty for the party. In 1992, as Castle was finishing up two terms as governor, he easily won a four-candidate House primary en route to winning the general election. Castle never faced a primary challenger in any of his eight re-election campaigns.

The field of potential candidates will be whittled down within the next few weeks, and GOP officials expect to have a consensus candidate soon, mindful that Carney has been campaigning for a long time.

“I do not think we’ll have a primary for the House,— Rakestraw said.

Kovach said Delaware Republicans “want to make sure that we have a full process and vet the strongest candidate to take on an opposition who has some name recognition— around the state.

“We want to find the strongest candidate who most represents core fiscally conservative, moderate values to represent the people of Delaware,— he said.

Asked about his own potential candidacy, Kovach said he’s been “flattered by the reaction that I’ve received— but that “there’s really nothing substantively new to report on that front.—

Republican officials said that whomever their party’s nominee is, he will be boosted by Castle’s presence at the top of the ballot.

“The party is fired up. Donors are enthusiastic. We certainly expect that there’s going to be a lot of support from the national party that will come in to help with the Senatorial race, and that’s going to have a trickle-down effect for all of our candidates, but especially the Congressional candidate,— said Seth Wimer, the executive director of the Delaware Republican Party.

But Pika expressed skepticism about the lift that Castle would provide to the GOP nominee for House. Castle has won crossover support from Democrats who haven’t backed Republicans in other Delaware races, he said.

“My suspicion is that people are so accustomed — those Democrats who are likely to cross over — to simply doing it for Mike Castle that I don’t think it’s going to carry over very far into the ticket,— Pika said.

Carney, meanwhile, has been keeping a low profile lately as he concentrates on preparing his campaign and raising money. Carney’s backers tout his high degree of name recognition and fundraising ability and his early start in a state that is voting more Democratic than Republican.

“While we’re taking nothing for granted, it’s clearly John’s race to lose,— said Dave Hamrick, a spokesman for Carney’s campaign.

Hamrick said Carney raised $2 million for his unsuccessful 2008 campaign for governor and has raised about half a million dollars for his House campaign.

“The Republicans have been unable to find a serious candidate to run in this race, which gives us a big head start in a Democratic district,— Hamrick said. “I think that all bodes well.—

With no candidate to promote yet, GOP officials are beginning to train their fire on Carney, suggesting that he’s not an insurmountable opponent because he lost to Markell in the 2008 gubernatorial primary and wasn’t selected by then-Gov. Ruth Ann Minner (D) to succeed Biden in the Senate. That appointment went instead to Ted Kaufman (D), a former Biden chief of staff who isn’t running in the 2010 race that Castle just entered.