THE NEW LITMUS TEST…. A whole host of Republicans will launch their presidential campaigns fairly soon, and they’ll do so knowing that the party’s voters have a variety of litmus tests in mind.

For example, if a candidate supports abortion rights, there’s no real point in even trying to win the nomination. If he or she believes someone, at some point, might need to see a tax increase by some amount, he or she will almost certainly lose. GOP presidential hopefuls who are respectful of gay rights should expect to do very poorly.

And in advance of 2012, it’s probably time to add a new issue to the list of litmus tests: only climate deniers need apply. The Republican tent, apparently, is only big enough to hold anti-science, anti-evidence leaders.

For some very likely presidential candidates, that’s a real problem.

It may be heresy to conservatives, but a trip down memory lane shows nearly all of the top-tier Republican presidential contenders want to save the planet from global warming.

On the campaign stump, in books, speeches and nationally-televised commercials, aspiring GOP White House candidates such as Tim Pawlenty, Mike Huckabee and Mitt Romney have warned in recent years about the threats from climate change and pledged to limit greenhouse gases. Some have even committed the ultimate sin, endorsing the controversial cap-and-trade concept that was eventually branded “cap and tax.”

Now, as they prepare for a wide-open primary season, many of the Republicans are searching for ways to explain themselves to a conservative voting base full of hungry tea party activists and climate skeptics who don’t take kindly to environmental issues so closely linked with Al Gore.

“They’re in an odd place,” Grover Norquist, president of Americans for Tax Reform, told POLITICO. “They better have an explanation, an excuse or a mea culpa for why this won’t happen again.”

Yes, in Republican circles in 2011, those who don’t reject the scientific consensus on the climate crisis will be rejected out of hand. Those who’ve been even somewhat reasonable on the issue in recent years should expect to grovel shamelessly — a trait that’s always attractive in presidential candidates.

The number of likely GOP candidates who’ve actually said out loud that the planet is warming and that human activity is responsible is, oddly enough, larger than the number of consistent climate deniers. Sarah Palin has said pollution contributes to global warming and “we’ve got to do something about it.” Romney has said he believes the planet is warming and at least used to support cap-and-trade. Huckabee and Pawlenty have backed cap-and-trade — which was, originally, a Republican idea, by the way — in recent years. Even Newt Gingrich used to demand “action to address climate change,” and participated briefly with Al Gore’s Repower America campaign.

This wasn’t a problem up until very recently. John McCain’s 2008 presidential platform not only acknowledged climate change, it included a call for a cap-and-trade plan — and he won the nomination fairly easily. As recently as 2006, rank-and-file Republican voters, by and large, believed what the mainstream believed when it came to climate science: global warming is real, it’s a problem, and it requires attention.

But that was before the GOP fell off the right-wing cliff. Now every Republican who was even remotely sensible on this “better have an explanation, an excuse or a mea culpa for why this won’t happen again.”

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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.