Rep. Mike Castle’s decision to run for Senate transforms the 2010 political landscape in Delaware and sets in motion what should be a titanic battle between the veteran Republican moderate and state Attorney General Beau Biden (D).

“This is going to be an exciting Senate race for Delaware, and one for the ages,— Sen. Tom Carper (D-Del.) said.

With Castle officially in the race, the focus now shifts to Biden, who is likely to run for the seat that his father, Vice President Joseph Biden, held for 36 years.

Where the GOP will emphasize Castle’s political independence and long experience, Democrats are expected to campaign on generational change in a race between the 70-year-old Castle and the 40-year-old Beau Biden.

Castle disclosed his political plans Tuesday in Wilmington, where he said, “We need the strongest and the most experienced leadership we can find in this country today.—

“I hope to be able to bring that to the United States Senate,— he said.

Castle’s decision was expected, even though he took a long time to arrive at it. He said he made up his mind a little more than a week ago but waited to announce until after Biden returned to Delaware from Iraq, where he had a military obligation as a captain in the state Army National Guard. Biden was elected to his office in 2006 and is up for re-election next year.

“I just got back from a year in Iraq,— Biden said in a statement. “I’m spending time with my family and I’m getting back to work. There will be time to make a decision.—

Appointed Sen. Ted Kaufman (D) is not running to serve the remaining four years of the term that the elder Biden won in November, simultaneous to his election as vice president.

Castle’s decision to run for the open Senate seat is a victory for GOP strategists in a Democratic-leaning state where he has been politically dominant. Party officials in Delaware and Washington, D.C., expressed confidence that Castle would help the GOP expand its 40-seat minority and also end its five-election losing streak in Delaware Senate races.

“On the national scale, clearly the Republicans need to add to their Senate numbers, and I think he’s the guy to get it done here in Delaware,— state Rep. Greg Lavelle (R) said.

National Republican Senatorial Committee Chairman John Cornyn (Texas) said the Senate campaigns of Castle, former New Hampshire Attorney General Kelly Ayotte and Rep. Mark Kirk (Ill.) demonstrate that “the Republican Party is a national party— when it attracts candidates like them.

“I think it’s a good signal to the country that Republicans want to be inclusive rather than exclusive, and we’re determined to rebuild our numbers and get back into the majority,— Cornyn said.

Castle had $861,000 in his campaign account at the end of June, all of which can be transferred to the Senate race.

The moderate Congressman never faced threatening Democratic opposition in his nine-term career. He topped 65 percent of the vote in each of his first six re-election campaigns, and even his more modest showings in the past two election cycles — 57 percent in 2006 and 61 percent in 2008 — were impressive, given the strong Democratic trend nationwide and in Delaware those years.

But Democratic officials signaled that they would tie Castle to the positions of former President George W. Bush and the national Republican Party, which still has a poor image nationwide.

“He built-up a record of supporting … George Bush’s economic policies, including tax cuts for the super-wealthy, that drove Delaware’s economy into a ditch — and now won’t support any of the Obama-Biden plans to fix it,— Eric Schultz, a spokesman for the Democratic Senatorial Campaign Committee, said in a statement. “This will be a race about who is positioned to lead Delaware into the future, and Democrats fully intend to hold onto the Vice-President’s seat.—

John D. Daniello, the chairman of the Delaware Democratic Party, said he has “a lot of respect for Mike Castle,— but he “has been growing increasingly out of touch with Delaware voters.—

Carper said that Castle “will be a very formidable candidate— but that “Democrats will have an equally formidable candidate of our own.—

A Rasmussen Reports poll conducted Sept. 30 suggests that a Castle-Biden race would be competitive: Castle had 47 percent and Biden had 42 percent.

“I don’t have any illusions that this will be easy street,— Castle said. “I realize this will be a very difficult election, and I’m sure the other side will be thinking the same thing.—

Although Castle’s announcement is good news for Senate Republicans, it complicates the party’s effort to retain the lone House seat in a state that has voted Democratic in the each of the past five presidential elections. The likely Democratic nominee, former Lt. Gov. John Carney, began campaigning for Castle’s seat in April and raised $262,000 through the end of June.

“We were targeting this district regardless of whether or not Congressman Castle decided to run for re-election,— said Shripal Shah, a spokesman for the Democratic Congressional Campaign Committee.

Andy Sere, a spokesman for the National Republican Congressional Committee, acknowledged that “Delaware’s political terrain presents challenges for the GOP— but predicted that with Castle leading the ticket, “a strong Republican will emerge to build on his legacy in the House.—

A long list of potential Republican candidates emerged in the immediate aftermath of Castle’s announcement. They include state Rep. Tom Kovach; former state Sen. Charlie Copeland, who was the party’s 2008 nominee for lieutenant governor; Colm Connolly, the former U.S. attorney for Delaware; Anthony Wedo, an investment and restaurant executive; and Lavelle, who said Tuesday morning that he “won’t rule it in and won’t rule it out.—

Kovach said he was “flattered— that Castle mentioned him as a possible candidate but added that he hasn’t yet made any decisions about the race. Kovach suggested the Republicans needed to field a candidate in Castle’s political mold.

“We need another independent-minded person to replace Mike Castle and to serve as a steward for Congressman Castle’s independent seat in Delaware,— he said.

Kathleen Hunter contributed to this report.