You probably couldn’t pick him out of a lineup of two, and for years, he has toiled in obscurity. But this week’s Senate debate over a health care reconciliation bill is likely to come down to one mustachioed man — Parliamentarian Alan Frumin.

[IMGCAP(1)]He is the go-to guy for rulings on whether the bill and any provision in it can pass with just a simple 51-vote majority or will need a 60-vote supermajority for success, and no one in either party, it seems, wants to upset him.

If Frumin says any part of the bill violates strict reconciliation rules, it will likely be stricken, because Senate Democrats do not have the 60 votes needed to retain it.

And if that happens, Senate Democrats will be forced to go hat in hand to House leaders with a plea for them to reprise their contentious and highly political Sunday night vote on the measure — a scenario that is sure to ignite further controversy and enhance the mistrust that exists between House and Senate Democrats.

Because reconciliation cannot be filibustered, the Senate GOP strategy has all along been to find at least one weakness in the bill that would force changes and a revote in the House. And Senate Republicans have repeatedly warned House Democrats that the likelihood of that happening is high.

Despite their protestations to the contrary last week, Senate Democrats finally acknowledged as much once the House had done its part. House Democrats narrowly approved the Senate version of health care reform legislation and the companion reconciliation bill late Sunday. President Barack Obama plans to sign the Senate piece of the package today.

The Senate battle for Frumin’s approval has been going on for weeks as House and Senate Democrats wrote the reconciliation measure, but Frumin refused to make some decisions until he had heard the Republican side of the argument. That process began in earnest this week.

[IMGCAP(2)]While ominous thunder rumbled outside, Republican and Democratic staffers met with Frumin and his staff at high noon Monday in an ornate Senate room and argued their positions for an hour. The topic was whether changes to a new tax on high-cost “Cadillac” insurance plans met the strict rules governing reconciliation. At press time, it was unclear whether Frumin had made a definitive ruling on whether the provisions violated rules that bar reconciliation from altering Social Security.

But that meeting is just the tip of the iceberg. A longer and more thorough meeting to vet the bill is expected as soon as today, once the Congressional Budget Office and the Joint Committee on Taxation have completed their analysis of changes made by the House before passage. After that, Republicans and Democrats said Frumin would likely be asked to make quick rulings throughout the week that could affect the outcome of the debate.

Most of the GOP challenges to the reconciliation package are expected to come from things they believe fall outside rules that bar provisions from being “extraneous” as well as those requiring reconciliation provisions to have a budget impact.

Senate Budget ranking member Judd Gregg (R-N.H.) described meetings with the Parliamentarian as a “point-counterpoint.”

“The staff sits in front of the Parliamentarian and makes their case. It’s just a nice, friendly discussion,” Gregg said last week. “Often, he’ll tell you where he’s going, but [Frumin] usually says he wants to think about it.”

Gregg also said that Members generally take care to not ambush the Parliamentarian with new arguments on the floor.

“Nobody wants to surprise anybody if we can avoid it,” Gregg said. “The Parliamentarian deserves to have a reasonable amount of information so he can make a thoughtful decision, and he’s not having to take action in a situation where he hasn’t had time to analyze the issue.”

Gregg indicated, however, that not every Republican will abide by that.

Frumin is no rookie when it comes to Senate procedural battles: He first joined the Parliamentarian’s office in 1977. His initial run as Parliamentarian began in 1987 when he was appointed by the then-majority Democrats. But when Republicans took over in 1995, Frumin was demoted and Bob Dove was elevated to the top spot. Dove was ousted by then-Senate Majority Leader Trent Lott (R-Miss.) over his rulings on reconciliation in 2001, and Frumin took over once again under an agreement between Lott and former Democratic Leader Tom Daschle (S.D.).

Given the high stakes of Frumin’s rulings in the current reconciliation debate, Democrats complained that Republicans are trying to pressure him by supersizing his role as referee in the process.

“Republicans want to elevate Alan Frumin and isolate him,” said one senior Senate Democratic aide. “They’re trying to put him on the defensive. … But Alan Frumin is as straight a shooter as they come.”

However, as Republicans have ramped up their procedural case against the reconciliation bill, they’ve softened their rhetoric about Frumin. Twice in the past four months, Republicans have accused the Parliamentarian of bias when he has ruled against them.

During the Senate health care debate late last year, Republicans said Frumin created a new precedent when he ruled that Sen. Bernie Sanders (I-Vt.) could withdraw his amendment after Sen. Tom Coburn (R-Okla.) had already forced a full reading of the 767-page proposal. Senate rules appear to say that only a request to waive the reading of a bill or amendment is in order during the reading process, but Democrats said Frumin’s ruling rested on a 1992 precedent.

Then again in January, Coburn cried foul when Frumin ruled he could not use an esoteric procedural tactic known as the “clay pigeon” during the debate on an increase in the debt limit. Frumin reportedly told Coburn that he could not use the delaying tactic on amendments that are part of a unanimous consent agreement limiting the overall number of amendments.

“You can’t do that. You can’t do that. Two times in a row you’ve set precedent,” an angry Coburn reportedly said to Frumin on the floor at the time.

But now Coburn is taking a slightly different tack.

“We’re probably less concerned about that,” said Coburn spokesman John Hart when asked about Frumin’s impartiality. “Dr. Coburn has obviously disagreed strongly — and still disagrees — with some of Mr. Frumin’s rulings, but he believes the Parliamentarian wants to faithfully, accurately and fairly interpret the rules. Mr. Frumin understands how his rulings will impact not only his professional reputation, but, more importantly, the Senate as an institution.”

Hart continued: “Vice President [Joseph] Biden presents a far more serious threat to the Senate than Mr. Frumin. Biden could use his own nuclear option and overrule Mr. Frumin’s rulings if he believed the bill was threatened.”

Indeed, Democrats on Monday would not rule out having Biden — in his capacity as Senate President — overrule Frumin’s decisions on whether a provision meets reconciliation rules. However, they said the likelihood of that happening is low, and that they are largely resigned to the fact that some minor provisions may be eliminated from the bill.

Democrats have already made it known that they are prepared to enlist Biden to enforce an obscure rule against “dilatory” amendments to shut down Republicans if the minority party appears to be unnecessarily dragging out the debate. The rule has never been used on a reconciliation bill, which otherwise cannot be filibustered, but it is unclear whether they would have Frumin’s blessing in using it.

Beyond the obvious potential pitfalls of Frumin’s rulings, Democrats said he could change the course of the debate by ordering something stricken. Some Democrats said a decision to strike a provision from the bill could create a ripple effect among their own rank and file.

“Once you make changes to the bill by taking something out, then Members have a big temptation to put in whatever their pet provision is, and that is something we’ll have to ward against,” said another senior Senate Democratic aide.

The Parliamentarian’s office declined to comment.