Nine-term Delaware Rep. Mike Castle, who lost his state's Senate GOP primary, said Wednesday that he won't endorse a candidate in the general election. 

Castle, who is a centrist, stunningly lost to Tea Party-backed candidate Christine O'Donnell in a bitter, hard-fought race. The congressman and former governor said last month that he would not endorse O'Donnell, raising speculation that he could back Democratic nominee Chris Coons.


"No, I'm not going to endorse anybody in that particular race, not because of the competence of any of the candidates, but because the primary I went through was very nasty in a variety of ways, both politically and personally, and I've just declined to get involved in that," Castle told CNN's John King in an interview to air Wednesday evening.

Castle's announcement comes just before Coons and O'Donnell are scheduled to square off in a nationally televised debate Wednesday evening on CNN.

Ever since the primary campaign, Coons has spoken highly of Castle, presumably with the hope that he would earn his support.

The New Castle County executive might not need it: most polls show Coons leading O'Donnell by double-digits in the race for Vice President Joe Biden's old Senate seat. 

But Democrats aren't taking the race for granted, Republicans had viewed the race as a key pickup opportunity in their quest to take control of the upper chamber.

President Obama and Biden are traveling to Delaware on behalf of Coons later this week for their first joint campaign appearance of 2010.

O'Donnell originally struggled to gain the support of national Republicans, who had largely backed Castle, but GOP leaders in Washington quickly circled the wagons around her.

Castle credited Tea Party groups for running an effective primary campaign and said they could be a formidable force in the general election as well.

"I will give credit to the Tea Party Express people who backed Christine O'Donnell. They're good politicians," he said. "Anyone who says they're not politicians doesn't know what they're talking about as far as I'm concerned. They did an effective job of winning the race for her and, you know, they're probably going to be very effective in setting up the last few weeks of the general election as well, if I had to guess."