The congressman hails from a key early-voting state. (Photo by Ricky Carioti/The Washington Post)

Rep. Mick Mulvaney (R-S.C.) has endorsed Sen. Rand Paul (R-Ky.) for president, saying that the "complete and utter surrender, the lack of conviction" from the House GOP's leadership forced his hand.

"The last two weeks have been probably the most frustrating weeks in Washington for me, and that’s a high bar," said Mulvaney, who has represented a safe conservative district since his victory in the 2010 Tea Party wave. "There’s no way for me to avoid that the establishment wing of our party has lost its way. They don’t have any idea the damage they’re doing to the Republican Party. I called up Rand and said, I’ve had enough. We’re either going to figure out how to save this party or the establishment is going to drive it to irrelevance."

Mulvaney's endorsement gives Paul a much-needed boost in perhaps the least habitable early primary state. In an average of polls, Paul runs 11th, with just 2.3 percent of Republican voters' support. Paul's summer decline in Iowa and New Hampshire led to a crop of "whatever happened to..." stories. The decline in South Carolina came from a lower start -- no poll this year has put Paul above single digits. And his father, former Texas congressman Ron Paul, put up weaker numbers in South Carolina than in any of the other early primary states.

Yet Mulvaney, an iconoclastic conservative who frequently breaks with leadership, said South Carolina was readier than ever for Paul's message.

"I think people, over the last four years, have seen that an interventionist foreign policy may not be the healthiest plan for our nation," said Mulvaney. "I will never ever forget the Vietnam vets that came into our office to talk about this -- especially this one giant man, this mountain of a guy, who said to me 'Look, we have to get our troops home. It’s killing our families. There are kids on their fourth or fifth deployments. It’s not isolationist, it’s not even close. It’s not even close to pacifist. It’s about a more circumspect, thoughtful use of military power. That’s falling on receptive ears. Our South Carolina sons and daughters are fighting these battles, and they don’t want to be used as decoys."

On the trail, several candidates who've never held office have gone after the same voters as Paul. Mulvaney ruled them out because "to really understand how broken the system is you need to have seen it from the inside just a little." And Mulvaney decided against endorsing Sen. Ted Cruz (R-Tex.), who has critiqued Paul's approach to the Planned Parenthood fight, because he doubted Cruz's effectiveness.

"I’ve seen both of them work," said Mulvaney. "When I see Ted – it’s almost as if Ted thinks if you yell loud enough and give a dramatic speech, it’s going to solve things, and I just don’t think that’s the way the world works. Here's the other thing I see out on the trail. Both Rick Perry and Rand Paul came to help me in the last cycle. About 400 people showed up both times. We knew that 40 people who had never come to me came to see Perry. We know that 250 of the people who came to see Rand had never been to one of my events. Ted is not bringing anybody new into the party. Rand is."

Paul, who joined most of the GOP field last week at a Greenville, S.C. policy summit, thanked Mulvaney for the support.

"It’s a big deal to get Mick Mulvaney’s endorsement," said Paul. "He’s one of the conservative leaders in the House. I think this endorsement will go a long way to showing I am the conservative candidate in this race."

The one caveat: Mulvaney's endorsement record. In 2012, he got behind the candidacy of then-Texas governor Rick Perry, a late entry to the race who promised to unite conservative voters. Perry imploded after a series of humiliating mistakes. Rep. Trey Gowdy (R-S.C.), who's staying neutral, has (jokingly) cited that experience as a reason why. He's fond of telling crowds that Mulvaney endorsed Perry "because he hates large crowds."

"Tell him that line was funny the first 400 times," laughed Mulvaney. "It’s easy to endorse somebody when they’re in first place. People were falling over themselves to endorse Trump. Those of us in politics are notorious for sticking our fingers in the wind. I'm not doing that. I’m so frustrated that if this can help, I’ll do it."