PARTY RALLIES BEHIND CANDIDATE IT CAN’T STAND…. Almost immediately after Christine O’Donnell had won the Republican Senate primary in Delaware, the National Republican Senatorial Committee made one thing clear: it had no intention of investing resources in O’Donnell’s general-election campaign.

About a half-day later, wouldn’t you know it, the NRSC has had a change of heart.

In a clear sign of the grassroots pressure on Republican leaders, National Republican Senatorial Committee Chairman John Cornyn just put out a statement embracing Christine O’Donnell — a dramatic contrast with his group’s curt response last night — and writing her a big check.

It’s a remarkable reversal, and a vivid illustration that the base is in charge and has the leadership running scared.

Cornyn not only endorsed O’Donnell, he also sent her a check for $42,000 — which may not sound like much in a statewide race, but it happens to be double the amount O’Donnell had in the bank as of two weeks ago.

Hot on the NRSC’s heels, former Gov. Mitt Romney (R), still desperate to impress far-right activists in advance of another presidential campaign, also endorsed O’Donnell. His political action committee also sent her a $5,000 contribution.

It’s a reminder that Republican leaders continue to go to great lengths to satisfy their base, but it’s also further proof of how the parties deal with fringe candidates who unexpectedly win statewide primaries. When Alvin Greene managed to win the Democratic Senate primary in South Carolina, leading Dems, including House Majority Whip Jim Clyburn of South Carolina, said they wanted nothing to do with him. It was a responsible thing to do.

But on the other side of the aisle, candidates who are nearly as ridiculous as Alvin Greene keep winning Republican primaries, and keep receiving their party’s backing.

No one seriously believes the NRSC has a patriotic duty to endorse Chris Coons, Harry Reid, Jack Conway, et al. But it is interesting that Dems distance themselves from their fringe, unqualified Senate candidates, while Republicans aren’t interested in doing the same.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.