The Post's Jose DelReal explains why Affordable Care Act architect Jonathan Gruber's year-old comments about the historic law have Republicans so angry. (Jose A. DelReal and Nicki DeMarco/The Washington Post)

What was Jonathan Gruber’s role in putting together the Affordable Care Act?

That question has been hotly debated in light of recently circulated remarks that the MIT economics professor made about the “stupidity of the American voter” and the “lack of transparency” that was required to secure the passage of the health-care law.

President Obama is the latest to play down Gruber’s involvement. “The fact that an adviser who was never on our staff expressed an opinion that I completely disagree with in terms of the voters is not a reflection on the actual process that was run,” he said Sunday.

While it is probably overstating things to describe Gruber as an “architect” of the law, as so many recent reports have, he was also no ordinary adviser — as evidenced by the fact that he was paid nearly $400,000 by the administration for his work.

And his advice was important at critical moments when the bill’s survival was in jeopardy.

One of those times was July 20, 2009.

Four days before, Congressional Budget Office Director Douglas Elmendorf had declared that the bills that were going through the legislative process in the House and Senate would fail to bring the “fundamental change” necessary to bring down health costs over the long run. Elmendorf’s pronouncement struck at one of the basic rationales for the whole endeavor of overhauling the nation’s health-care system, even as opposition was building on the right.

Gruber was among a small group of economists that the president summoned to the Oval Office to meet with him and Elmendorf. The other two were former CBO director Alice Rivlin and Harvard University health economist David Cutler. They pored over the bill to look for other, more credible ways to wring out savings.

Drawing on that conversation, Obama met the following day with moderate "Blue Dog" Democrats, who had been holding up the bill in the House. He urged them to revisit an idea that had previously been rejected by committee chairmen on Capitol Hill. It would take from Congress the power to set Medicare reimbursement rates — a major driver of overall health-care costs — and put it in the hands of an independent board. That, many analysts believed, could be a “game changer.” It also remains one of the most controversial elements of the new law.

Gruber’s influence derives from the fact that he has developed a "microsimulation" computer model that can produce estimates of the cost and effects of health-care policy changes more quickly than anyone else can. Politicians of both parties have relied on him.

He had been a student of future treasury secretary Larry Summers at Harvard and worked a stint in the Clinton administration before returning to academia. The idea for developing the computer model, he told me in 2009, had actually come from Bill Clinton health care adviser Chris Jennings, who would later play the same role in the Obama White House.

“What people in Washington need,” Gruber recalled Jennings telling him, “is someone who is objective and willing to give them answers, not ‘on the one hand, on the other hand.’”

“The fact that I was an academic willing to do this put me in a unique position,” Gruber said.

His role, however, was not to set policy. It was to explain the effect that a policy choice would have and to add credibility to the entire endeavor. That was how he found himself so close to the decision-making that day in the Oval Office — and why he is on the hot seat now.