Opinions vary as to what, exactly, caused Rick Perry’s precipitous fall from “frontrunner” to “struggling second-tier afterthought,” but I’d argue the collapse was the result of one fleeting moment of sanity from the Texas governor. In September, Perry endorsed a sensible immigration policy, suggested his opponents don’t “have a heart,” and immediately saw his support plummet.

The key takeaway from this is that immigration remains fairly radioactive in Republican politics. With that in mind, as John Dickerson explained this morning, Newt Gingrich took a major risk in last night’s debate.

At the CNN national security debate on Tuesday, the former speaker said that he would not be in favor of kicking out illegal immigrant families that had been in the country for a long time. “The party that says it’s the party of the family is going to adopt an immigration policy which destroys families that have been here a quarter century?” he said. “I’m prepared to take the heat for saying, ‘Let’s be humane in enforcing the law.’” […]

After the debate Gingrich stuck to his position on immigration, the broader shape of which is based on a “red card” program put forward by the Krieble Foundation. “Millions will go home,” he said after the debate, “but there will be millions who will be staying.” He said no one should kid themselves about the unworkability of deporting 11 million people. He also made his case on the grounds of simple human kindness. This, almost exactly, was Ronald Reagan’s position. In a 1984 debate with Walter Mondale, the Republican icon said: “I believe in the idea of amnesty for those who have put down roots and lived here, even though sometime back they may have entered illegally.”

Of course, by 2011 standards, Republicans consider Reagan a borderline-socialist sell-out, so it’s not as if Gingrich can rely on Ronaldus Magnus for cover. Indeed, Michele Bachmann immediately went on the offensive, as did Mitt Romney and his campaign.

As is often the case, Romney’s team was pressed on what policy, exactly, the former Massachusetts governor would prefer as an alternative to Gingrich’s “humane” approach, but the Romney campaign struggled to answer.

Regardless, how big a problem is this for Gingrich? Time will tell, of course, but I’d be surprised if we saw a Perry-like collapse. For one thing, Gingrich simply explained his position more effectively than the Texas governor did, and didn’t condemn those who disagreed. For another, there are subtle-but-significant policy differences between Gingrich’s approach and Perry’s.

But there’s also the fact that the campaign is simply in a different phase than it was in September, and the shrinking calendar is likely to affect the party’s reaction. Not only are there no other viable non-Romney candidates for anti-immigrant candidates to flock to, but this is about the time voters are more likely to weigh general-election electability considerations.

Still, there’s no denying Gingrich gambled by saying something reasonable, especially since Iowa will be so important to his campaign and anti-immigrant animus runs strong among Hawkeye State Republicans. It’s worth keeping a close eye on this.

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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.