Republican vice presidential candidate Paul Ryan’s signature achievement in Congress is his plan to bring runaway federal spending, particularly the skyrocketing cost of Medicare, under control. With Ryan under Democratic fire for his Medicare proposals, columnist Charles Krauthammer suggested Ryan simply declare his plan “history” and move on. “The details of the Ryan plan have some things that don’t mesh with what Romney is running on, and are a little harder to defend,” Krauthammer said Tuesday on Fox News. “I think what Ryan has to say is, look, I proposed the plan in Congress, a congressional plan. It was rejected by the Democratic Senate and by the president. It’s now history.”

Ryan is already doing that, at least in part. In his interview with Fox’s Brit Hume Tuesday, Ryan said he has abandoned his plan’s provision to take about a half-trillion dollars out of current Medicare funding in order to shore up the Medicare trust fund for the future (as opposed to the provision of Obamacare that takes about $700 billion out of Medicare to pay for health coverage for currently uncovered people). Romney’s proposal is not to take the money out at all, so Ryan has abandoned his old position and now supports Romney’s.

“I joined the Romney ticket,” Ryan said. When Hume pressed, Ryan said, “[Obama] is taking those dollars from Medicare to spend on Obamacare. We prevent that from happening in the House budget, but let’s be clear here, Brit. I am on the Romney ticket. And what Mitt Romney is proposing is to repeal all of Obamacare.”

If anyone missed the point, Romney himself hammered it home Wednesday morning in an appearance on CBS. Asked about Ryan’s Medicare plan, he said, “First of all, Congressman Ryan has joined my campaign, and his campaign is my campaign now and we’re on exactly the same page, and my campaign has made it very clear. The President’s cuts of $716 billon to Medicare, those cuts are to be restored if I become president and Paul Ryan becomes vice president.”

The Ryan and Romney appearances left a question: How much of the Ryan plan will Ryan have to abandon to bring his positions in line with those of Romney? The answer is some — but not a lot. “There’s not much you’d take out, because Romney hasn’t been nearly as specific,” says one Washington policy wonk deeply familiar with Ryan’s plan. “There are a lot of places where Romney just hasn’t said what he would do.”

Medicare cuts are clearly one area. Taxes are another — sort of. “Their tax plans are a little different, but neither of them really has a tax plan,” says the wonk. It is Romney’s lack of specificity, the wonk says, that ensures there won’t be many conflicts between Romney and the Ryan plan. On the other hand, that leaves the possibility that Ryan will have to back away from some of his specifics because they might conflict with future Romney positions.

And then there are the House Republicans. Ryan can back away from some elements of his plan, but what about them? They voted for it — nearly unanimously. What position should they take on, for example, the Medicare cuts for which they have already voted? “They say, ‘We passed the budget, here’s where we’re at, we’re now in a debate that is different from where we were in the spring,” says one well-connected Republican.

Another GOP insider says that will work only if Ryan’s departures from his budget are relatively minor. “I don’t think there can be any wholesale backing away from what we supported,” says the insider. So far, the insider doesn’t see a major problem. But what he suggests indicates the problems Ryan might have with the Krauthammer approach. He has invested too much in his budget plan to back too far away from it now.