A conference call from the Romney campaign on Thursday may prove to be a critical moment in the 2012 presidential campaign.

The subject of the call was Paul Ryan’s controversial proposal for Medicare – and the very different ways that Romney and Newt Gingrich, have treated it. Ryan’s plan, as you may recall, is controversial because it would end Medicare as we know it, transforming the government insurance plan into a voucher scheme with no guarantee that the vouchers would enable seniors to buy comprehensive insurance. It would save the government money, in theory, but only by leaving seniors exposed to much higher individual health care costs. (Here's a quick refresher on that from Igor Volsky, if you need one.)

After Ryan unveiled his plan, Romney praised the Ryan plan while Gingrich condemned it as “right-wing social engineering.” The point of the Romney campaign conference call was to highlight that contrast. It's part of a broader, furious attack on Gingrich's conservative credentials, now that Gingrich is ahead of the polls. “Mitt Romney supports what Paul Ryan did. He endorsed what Paul Ryan did,” former New Hampshire Governor John Sununu, a Romney supporter, said on the conference call. Later Sununu added, “Gingrich’s undercutting of Paul Ryan proves that he is more concerned about Newt Gingrich than he is about conservative principle.”

Romney has sketched out his own ideas for changing Medicare and they look similar to Ryan’s, in that Romney would transform Medicare into a voucher scheme. But Romney has also said there’d be a few differences: Chief among them, Romney would preserve Medicare as an option for seniors, something Ryan would not. Romney has also been vague about exactly how much the vouchers would be worth. That's a key point, since a major problem with the Ryan plan is that the vouchers are so skimpy – although, of course, that's also the reason for its budget savings.

Presumably Romney's lack of detail on the issue had some political motive: Romney wanted to avoid taking positions that might alienate seniors, who, as Sarah Kliff points out, get very nervous about changes to Medicare. But Sununu went out of his way during the conference call to emphasize the similarities. “Mitt Romney had his own package of entitlement reform, which Paul Ryan has praised. They both meshed together. They are both based on really understanding entitlement reform.”