Mitt Romney has picked up an unexpected endorsement: that of Tea Party darling Christine O'Donnell, who upset establishment Republican Mike Castle in a 2010 congressional primary in Delaware before losing to Democrat Chris Coons in the general election.

I think infrastructure and executive experience are important, and for that reason I'm endorsing Mitt Romney, O'Donnell told Fox News host Sean Hannity on Tuesday.

O'Donnell is not a power player within the Republican Party, but her endorsement is significant because Romney has struggled to win over Tea Party supporters, many of whom distrust him for the health care law he passed as governor of Massachusetts -- dubbed Romneycare to liken it to Obamacare -- and for his flip-flops on a variety of issues.

O'Donnell gave an interesting response to that criticism: He's been consistent since he changed his mind.

She said she actually admired Romney for being willing to admit that he was wrong and change his positions accordingly, and added that she was not supporting Romney reluctantly because he is the most electable or because she dislikes his opponents.

I am endorsing Governor Romney because I trust him to do the right thing, she said. He has the strength of conviction to do the right thing regardless of opposition.

This is not anti-Gingrich or anyone else. It's a pro-Governor Romney endorsement, she added, referring to former House Speaker Newt Gingrich, who is leading the polls among Republican primary voters.

O'Donnell did, however, have harsh words for Gingrich.

People are trying to paint Newt Gingrich as the anti-establishment candidate, which I think is funny because in a lot of the Tea Party vs. establishment campaigns in 2010, Newt Gingrich was on the side of the establishment. The Tea Party I don't think should be behind Newt at all, she said. If you want to get behind Michele Bachmann, you want to get behind Rick Santorum, Ron Paul, instead of Romney, OK, I can understand that. But not Newt Gingrich.

She said that Romney, despite his two presidential campaigns, was not a Washington insider.

Romney touted the endorsement in a statement.

Christine has been a leader in the conservative movement for many years, he said. Christine recognizes that excessive government threatens us now and threatens future generations, and I am pleased to have her on my team.

It is the latest in a string of prominent endorsements for Romney. His supporters include New Jersey Gov. Chris Christie, former Minnesota Gov. Tim Pawlenty, former Vice President Dan Quayle, Sen. Kelly Ayotte of New Hampshire and Sen. John Thune of South Dakota.