Now that the 2012 GOP field has been winnowed to only two candidates with any real chance at the nomination, the next big question is this: Who will key conservative leaders and groups support?

The answer will determine whether Mitt Romney will waltz to the nomination, probably wrapping it up later this month — or if we’re in for a long, tough struggle, with Rick Santorum having a fighting chance.

Exiting Iowa, Romney has a huge lead in high-visibility endorsements, which political scientists have shown are a good predictor of who will eventually get the nomination. But a large number of Republican politicians, interest group leaders, and other prominent party players have held back this time. The Washington Post list of key endorsers shows that only 10 of the 50 have said anything so far, and three of those went to Rick Perry, who is likely dropping out soon.

Presumably, most of the moderate leaders will eventually wind up with Romney. The real question is what conservatives will do. Watch people such as South Carolina Senator Jim DeMint; Oklahoma Senator Jim Coburn; former presidential candidate Mike Huckabee, and a bunch of the new Governors and Senators elected in 2010. Watch, too, conservative leaders and media stars such as Erick Erickson, Ralph Reed, Rush Limbaugh, Grover Norquist, and others. And of course there are the fallen candidates: Herman Cain, Rick Perry, Michele Bachmann, and Sarah Palin. Not just whether they endorse, but short of that how they treat the candidates and spin the news out of Iowa.

Because Romney has a large head start in the race in terms of resources, and because he will likely monopolize those who care primarily about how Republicans do in November, Santorum will have to do very, very well among these conservative leaders. If he does, they will both deliver resources to him (money, favorable publicity, volunteers) and send a signal to conservative voters that the former Pennsylvania Senator is the obvious conservative choice — and that Romney should be seen as unacceptable to them. If the message from them is more muddled, then Romney will likely split the conservative vote and lock up the nomination quickly.

After New Hampshire, where Romney will likely win but where Santorum should finish third or better, knocking out Jon Huntsman, the big tests will come in South Carolina on January 21 and in Florida on January 31. So if conservative party actors want to shut this thing down in favor of Romney and avoid a drawn-out battle, the time for them to act is now. If they begin to break towards Santorum, or even if they remain silent, it could be trouble, signaling that they are unwilling to stamp the conservative seal of approval on Romney and aren’t prepared to tell conservative rank and file voters that the time has come to settle for him.