After months of work involving nearly one third of the House and half the Senate, prospects for healthcare reform are coming down to what six senators can work out by a self-imposed Sept. 15 deadline.

For House Democrats, it’s a baffling calculus – that so few would matter so much. But it’s a situation they have seen before.

Three GOP senators were the swing votes in the Obama administration’s economic stimulus plan in February. In 2001, three Republican senators withheld their votes until President Bush reined in his tax cuts.

Now, the essential swing votes on healthcare sit in the Senate Finance Committee, where three Republicans and three Democrats have been thrashing out a bipartisan alternative behind closed doors for weeks.

“As long as you have an institution where the filibuster is used so regularly, then a handful of people will sway the debate,” says Julian Zelizer, a congressional historian at Princeton University in New Jersey. “It’s been congressional politics for the last two decades.”

The Senate Health, Education, Labor, and Pensions (HELP) Committee and the House Education and Labor Committee have already passed nearly identical versions of a government insurance plan – a top priority for President Obama and most congressional Democrats. These bills include a government-run healthcare option designed to compete with private plans.

But this public option is a nonstarter for the bipartisan negotiators on the Senate Finance Committee.

The clout of the few – and particularly the importance of the Senate Finance working group – can be a source of frustration for the many on Capitol Hill.

“It’s no secret that [House] members sometimes think: Why do I always read in the paper that they’re checking with the [Senate] Finance Committee all the time,” Speaker Nancy Pelosi told reporters Friday.

“What about the HELP committee? What about our committees over here?” she added.

Sens. Olympia Snowe of Maine and Charles Grassley of Iowa and Michael Enzi of Wyoming are the three key Republicans in the Senate Finance group. Senator Enzi's role in shaping potential healthcare legislation is instructive of the influence that a handful of senators can wield.

As the top Republican on the Senate Health Committee, he could do nothing to stop the committee from passing a bill that included a public option in a party-line vote. But Senate Democrats sense that they might need to pick off some Republican votes to get to a filibuster-proof 60 votes on healthcare. So the Gang of Six’s effort to find bipartisan compromise has become vital – and Senator Enzi can make his stand there.

Aides this week say that there are three potential deal-breakers: Healthcare reform must be fully paid for, with no new debt put on the backs of Americans. Everyone must get coverage. And robust competition on the delivery side must be preserved.

What this means is that “a government option that can drive private service providers out of business, either by cutting reimbursements or peeling away subscribers currently enrolled in private plans, can't be part of healthcare reform,” says Enzi spokesman Craig Orfield.

But he adds that "there's a long way to go here, and many of the issues have only been discussed as concepts."

As an alternate route, Senate Democratic leaders are considering the option of using a process called budget reconciliation, which would allow them to pass healthcare with a simple majority vote.

But it’s an iffy prospect, with many procedural pitfalls. Even if Democrats could make it past procedural issues, the end result would be a partisan bill.

“The threat is that it wouldn’t last over time: Republicans would chip away at it as soon as they had power again. That’s the great risk,” says Professor Zelizer.

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