Rep. Mike Castle (R-Del.) will run for the Senate seat once held by Vice President Joe BidenJoe BidenThe Hill's 12:30 Report Biden spotted at Wizards playoff game Trump’s wall jams GOP in shutdown talks MORE, he announced in a noon press conference in Wilmington.
After months in which he went back and forth over whether to run for the remaining four years of Biden's term or retire at the end of a long career in politics, Castle said he would make the race.
"It has never been more important for Delaware to have the strongest and most experienced leadership to represent us in Washington," he added. "We are best served by an experienced, independent voice and that is why I am announcing my intention to run for the United States Senate."
Castle's decision gives Republicans a major recruiting win and a strong possibility of picking up a Democratic-held Senate seat for the first time since 2004. National Republican Senatorial Committee chairman John CornynJohn CornynSenate's No. 2 Republican: Border tax 'probably dead' McConnell: Senate will pass short-term funding bill to avoid shutdown The Hill's 12:30 Report MORE (R-Texas) said Castle's entry "instantly transforms" the race into one of the top races in the country.
It will also be expensive, Cornyn hinted. Castle "has a proven record of representing the people of Delaware in a bipartisan fashion, and we are thrilled that he has decided to take this next step toward extending his career of public service in the United States Senate. The NRSC will ensure that Mike Castle has all the necessary resources to win this seat next November," he said in a statement.
But Castle's entry will not guarantee the seat goes to Republicans. Castle will most likely face a tough challenge from Delaware Attorney General Beau Biden, the vice president's son, who has been rumored to be contemplating a bid while serving in Iraq over the past year.
"I don't have any illusions that this will be easy street," Castle said at his press conference.
No reliable polls have been taken recently, but strategists in both parties acknowledge that a Castle-Biden showdown would instantly become one of the most competitive races in the country. Both men are widely popular in the state.
Castle is a former two-term governor serving his ninth term in Congress. He is seen as a centrist and has been a leader of the Republican Main Street Partnership.
The younger Biden was first elected attorney general in 2006, and just returned from a yearlong deployment as a captain in the Judge Advocate General Corps with the Delaware National Guard.
Sen. Ted Kaufman (D), a former chief of staff in the elder Biden's Senate office, was appointed as a placeholder and has said he will not run for the seat.
Castle said he expected Biden to be his opponent next year, and that he expects a difficult contest.
"I have a lot of respect for Joe Biden and, more importantly, friendship," Castle said. "I like Joe and I like his family a great deal. That is a very political family, very confident in the art of politics. I realize that when you are dealing with any of the Bidens, you are dealing with all of the Bidens."
Delaware Democratic Party chairman John Daniello said his party would retain the seat Biden held for so long, despite the challenge Castle's candidacy presents.
"I have a lot of respect for Mike Castle. He has held a long and sometimes overstated career in public service," Daniello said in a statement. But, he added: "Rep. Castle has been growing increasingly out of touch with Delaware voters. He tried, but was relatively unsuccessful, at furthering his agenda under a Republican administration, so I question how much more effective he will be under a Democratic administration."
"I regret that Mr. Castle has given into pressure from the RNC to run, when I believe he preferred to retire, but I look forward to seeing a substantive debate on his record in the weeks and months to come," Daniello said.
Castle's decision puts to rest months of speculation about the Republican's political future. He had hinted he was extremely unlikely to run for his old House seat and said he was contemplating either a Senate bid or retirement.
And Castle played his cards close to his vest. Even on Tuesday, top Republican officials in Delaware and Washington did not know of Castle's ultimate decision before word leaked little more than an hour before his announcement.
Castle's departure from the House gives Democrats a strong opportunity to pick up another seat. Former Lt. Gov. John Carney (D) is already in the race, and many acknowledge that the GOP's chances of holding the seat are slim. But Republicans insist they will put up a fight.
State Rep. Tom Kovach and businessman Anthony Wedo headline a list of potential Republican candidates, but none of them packs the kind of name recognition as Carney.
GOPers see Kovach as more in the mold of Castle, and they are encouraged by his special election win in a tough state House district last year. Wedo is a political newcomer but would bring personal resources and business connections to the race.
Other options include former Assistant U.S. Attorney Ferris Wharton, though he would be more apt to run for Beau Biden’s office if Biden runs for Senate. Wharton fell to Biden by five points in their 2006 matchup for that office.
“With Rep. Castle leading our ticket in 2010, we are confident that a strong Republican will emerge to build on his legacy in the House,” said a spokesman for the National Republican Congressional Committee (NRCC), Andy Sere.
-- This article was updated at 12:40 p.m.