For Republicans running for president in 2012, there’s a new political reality: Support Rep. Paul Ryan’s budget plan — or else.

Newt Gingrich learned that lesson the hard way. After the former House speaker criticized the Medicare overhaul included in Ryan’s proposal as “right-wing social engineering,” conservative criticism rained down on him.

“Here you’ve got Representative Ryan trying to bring common sense to this world of insanity, and Newt absolutely cut him off at the knees,” South Carolina Gov. Nikki Haley said in an interview with CNN. (An Iowa voter agreed: He confronted Gingrich and told him that his comments about the Ryan plan were “unforgivable.”)

Gingrich (Ga.), sensing that he was losing GOP primary voters less than a week into his candidacy, called Ryan (Wis.) to apologize.

In the aftermath of the Gin­grich episode, former Utah governor Jon Huntsman was forced to get specific about the Ryan budget on Friday. He told ABC’s George Stephanopoulos he would have voted for the plan, “including the Medicare provisions.”

And Priorities USA Action, a political group run by two former White House aides, began airing an ad in South Carolina on Friday that criticizes former Massachusetts governor Mitt Romney, who was visiting the state, over the Ryan plan. “Mitt Romney says he’s ‘on the same page’ as Paul Ryan,” the narrator says. “With Mitt Romney, you have to wonder: Which page is he on today?”

There’s almost certainly more where that came from for the 2012 GOP contenders, who, before Gingrich’s struggles, had largely stayed away from engaging in the specifics of the Ryan plan — choosing instead to broadly praise its boldness.

“One candidate’s shakedown leaves the party shook up,” said Tucker Eskew, a longtime party strategist. “The reckoning was coming anyway, but this episode starkly reminds campaigns that entitlements are on the chopping block.”

If Gingrich’s experience is any indication, it’s now clear that, for many Republicans, criticizing what Ryan laid out — and the House approved — amounts to apostasy.

But it’s equally clear that, outside of a Republican primary electorate, supporting the changes in Medicare that Ryan has proposed is politically perilous.

In an Upstate New York special election to fill the seat of former congressman Chris Lee (R), Democrats and their allied outside groups have pounded away at Republican Assemblywoman Jane Corwin’s support for the Ryan plan. One ad being run by the Democratic Congressional Campaign Committee hits Corwin for backing legislation that “essentially ends Medicare.”

New independent polling suggests that the Medicare attack has political legs. Medicare was the most important issue for voters in the 26th Congressional District in a Siena Research Institute poll. More than one in four voters age 55 and older called it the critical issue of the special election. That same poll showed Erie County Clerk Kathy Hochul (D) leading Corwin 42 percent to 38 percent. Independent candidate Jack Davis received 9 percent.

If Hochul goes on to win in Tuesday’s special election, expect Democrats to seize on the results in the district — one of only four that Sen. John McCain (R) won in New York state in 2008 — as evidence of the deep danger of the Ryan plan for Republicans at the ballot box next fall.

Of course, for the likes of Gin­grich, Huntsman and Romney, the political calculation is more near-term and aimed almost entirely at a Republican primary electorate that has little interest in candidates expressing something short of full fealty to spending cuts and debt reduction.

That means the next six months of the Republican primary race are likely to feature candidates trying to outdo one another in displaying their commitment to fiscally conservative principles — a rhetorical trend that could complicate down-ballot GOP candidates’ attempts to distance themselves from the Ryan plan.

“I think candidates running for president should have a plan of action to deal with the entitlement time bombs and our deficits and debt,” said former Florida governor Jeb Bush. “That would include President Obama [who] has been missing in action.”

Said Gentry Collins, a former political director at the Republican National Committee: “The only way I can see around Ryan’s plan is to have one of your own that is every bit as serious. I don’t see most campaigns wanting to do that, which I actually think is an error in this environment, and therefore see them as having to be responsive to the Ryan plan.”