Why did Mitt Romney endorse S.C.'s Nikki Haley?

That's one of several questions raised by Karen Tumulty's great story Thursday on the politics of down-ballot endorsements by potential 2012 GOP presidential candidates.

Some Republicans were mystified by Romney's decision in March to back Nikki Haley, a South Carolina state representative, in the governor's race there. Those close to Romney note that she was one of the few prominent South Carolina Republicans to stand with him in that state's presidential primary two years ago. But Haley's a long shot in a four-way primary. It may turn out that Romney hasn't done himself any favors with the ultimate winner in a crucial primary state.

She may be a long shot, but Haley won the early and enthusiastic support of Erick Erickson of RedState, who then successfully sold her story to the conservative netroots. By summer of last year, Pulitzer-winning Post columnist Kathleen Parker -- who has a home in the state -- was touting Haley.

By endorsing Haley, Romney is declaring his solidarity with a tea party-attending candidate backed by conservative activists. If she loses, he retains that credibility and heads into South Carolina with the kind of rock-bottom expectations he can exceed. Because Romney remains a brutally hard sell to South Carolina GOP voters. It's one state where his northern roots and Mormon faith do him no favors.

In 2008 he spent more than $2 million on TV ads in the state to win 15.3 percent of the vote, carrying none of the state's counties