It is “sad,” says MIT professor Jonathan Gruber, that presidential candidate Mitt Romney is running away from his Massachusetts health care plan, a plan that Gruber says “gave birth to one of the greatest pieces of social legislation in our history,” namely President Obama’s 2010 health-care reform legislation. Aside from Romney, Gruber is the man most responsible for the Massachusetts plan. And given how important the debate is over the ‘Cares — RomneyCare and ObamaCare, as they’re often dubbed — I decided to go to the man who perhaps knows more than anyone about the development of both.

In a candid phone interview, Gruber explained how RomneyCare came about, where he thinks the critics have it wrong and whether potential “defenses” against his own plan by the Republican 2012 frontrunner hold up to scrutiny.

Gruber says that by the time he met Obama in 2006, the Massachusetts plan had already passed. He recalls that coming out of the Clinton years, “when the government had money to spend,” states could apply for grants from the Health and Human Services Department to study how the states could expand health care coverage. He was contacted by the Massachusetts department, which had received such a grant, and worked on different “models” for health care reform. When Romney later entered office and wanted to work on health care reform, the same department called Gruber in, because, of course, he had done much of the work already.

Gruber tells me, “What I can tell you is that there was an active debate on the individual mandate.” He explains, “On one hand, Romney felt people were free-riding” on the health care system — that is, remaining uninsured but burdening the government health care system when they became sick or injured. On the other hand, Gruber says, was the “freedom” argument about the mandate. He says, “What I’d like to think carried the day was that I pointed out that without the mandate you would spend the same money and cover fewer people.”

Gruber says in the one face-to-face meeting he had with Romney, it was clear Romney had made the “final call.” Gruber tells me: “He was the champion of the plan. He really was the consummate management consultant.” And, Gruber says, the plan that passed and is now operating is not fundamentally different from the individual mandate plan that Romney “championed.” Gruber says, “The framework is basically the same.” Moreover, Romney understood that the bill would be a framework, with the details to be filled in later (e.g., the definition of “affordable” health care). “He said so at the signing ceremony,” Gruber recollects. So the notion that the plan was hijacked by the Democratic-controlled legislature is simply not so, he asserts.

It is equally true in Gruber’s mind that without the Massachusetts example, Obama’s individual mandate plan in all likelihood would not have passed. He says that as the federal health care plan emerged, the Massachusetts plan was “widely discussed.” And he should know. He was first called in as an unpaid consultant to work on Obama’s health care plan, then as a paid consultant to HHS to work on health care modeling, and then as a paid consultant working with Congress to develop the bill. He wasn’t alone. John Kingsdale, the head of the Connection Board (the Massachusetts health care purchasing exchange) was a frequent presence on the Hill. Gruber says he was asked “dozens of times” about the Massachusetts plan. He tells me that in his view (and what he told lawmakers at the time) was, “It worked. People liked it.” In his opinion, without the Massachusetts plan the federal individual mandate plan wouldn’t have garnered acceptance and gotten through. “It was huge,” Gruber says, to have the Massachusetts plan to point to. And without it, he thinks “it’s likely” ObamaCare wouldn’t have become law.

Unlike conservative Republican primary voters and elected officials (who abhor the individual mandate), Gruber remains a strong defender of it and therefore finds it “depressing” that Romney now feels compelled to distance himself from it. He recollects, “I came out of that meeting with Mitt Romney as a Democrat . . . thinking he’d be the next president.” But that was a different Romney, a cheerleader for the individual mandate, than what we see today. In Gruber’s mind, “Romney should be proud” of his health-care reform. Gruber says that the plan did what it was supposed to do, namely “fix the insurance market” and cover more people. He says it didn’t control costs, but “it wasn’t supposed to.” (By contrast, that’s how Obama sold his federal plan.)

What about Romney’s position that the states should be allowed to do things differently? Gruber says, “I’m not opposed to state innovation.” But he says it’s not right to say that “Massachusetts did it on its own.” He observes , “Where are they getting the money?” From the federal government, in large part. So, he wants to know whether as president Romney would allow federal dollars to support state individual mandate plans.

Romney’s supporters sometimes advance the idea that the Democratic legislature piled goodies onto the plan. Gruber says this is true in one respect. “The safety net hospitals [the two biggest in Boston and in Cambridge] got a huge amount of money. . . . Romney wanted to rededicate that money to covering the uninsured.” However, Democrats later put back in the money to safety net hospitals, at a cost of perhaps $100 million. Romney apparently did not see that one coming.

In short, Gruber is the most powerful voice (and the most dangerous one for the Romney campaign) for three fundamental points: 1) Romney championed the individual mandate, overriding concerns about personal freedom; 2) the plan today is pretty much the same as what Romney signed into law; and 3) without it in all likelihood we wouldn’t have ObamaCare today. That might sound like a reason for Democrats to vote for him, but in a Republican presidential primary all of that may be the death knell of the Romney candidacy.