At least one other Delaware Republican was caught off guard by Christine O’Donnell’s endorsement of former Massachusetts Gov. Mitt Romney this week.

“Who endorsed Romney? Christine O’Donnell did? I’m surprised,” former Rep. Mike Castle, whom O’Donnell upset in the 2010 Senate primary, said Wednesday in a phone interview.

O’Donnell gave Romney her blessing in the presidential race on Tuesday night in an interview on Fox News, citing his commitment to her failed campaign against now-Sen. Chris Coons (D). O’Donnell, a three-time losing candidate, became a conservative cause célèbre during her insurgent campaign, which was marked by repeated media firestorms and unusual ads.

Nonetheless, Romney quickly touted her support in a press release. The Democratic National Committee mocked Romney by forwarding the release to reporters as an in-case-you-missed-it.

Castle, a fellow Romney supporter, said he’s been “quietly” supporting Romney since the start. He said he’s happy O’Donnell also supports the GOP’s “best chance of winning the presidential race.”

“I’m a little surprised because I don’t necessarily view Romney as the most conservative of the candidates out there,” Castle said. “And certainly, that’s the raison d’être of Christine O’Donnell’s campaign in the past. I’m pleased that she’s supporting him.”