Senate Majority Leader Harry Reid eked out 60 votes on a procedural motion to start the health care debate Saturday night - so there's no guarantee he can pass a bill on the merits.
And if he struggles, the reasons will be clear: deep divides among Democrats on a public insurance plan, abortion, tax hikes and cost-cutting. Liberals want the plan to be generous enough. Moderates fear a budget-buster. And everyone is trying to avoid angering seniors.
Even in the blush of Saturday's victory, Reid (D-Nev.) is far from having the votes to move his $848 billion package to final passage. At least four centrists have pledged to oppose it in its current form, largely over the public option. Reid is in a bind. Stay to the left, and moderates vote no. Move a tad to the right, and Reid faces insurrection from the left, as liberals in his own caucus and in the House vow not to compromise any further on their signature issue.
As one of the Senate most liberal members, Bernie Sanders (I-Vt.), told POLITICO's The Arena: "I have made it clear to the administration and Democratic leadership that my vote for the final bill is by no means guaranteed."
Health care reform proponents considered Saturday's vote a major milestone, one that significantly boosted the odds of passing a bill. But, as Senate Minority Leader Mitch McConnell (R-Ky.) vowed Saturday, "the battle has just begun" - and here are the battle lines where President Barack Obama's vision of reform could falter.
No good option for public option
A Democratic dream - expanding the government's role in guaranteeing health care to the uninsured - might well be reform's undoing.
Public option proponents, including Sanders and Sen. Sherrod Brown (D-Ohio), say they have already given up enough. They agreed to forgo a single-payer system. They decided not to push a government plan tied to Medicare rates. And they accepted Reid's proposal to include the opt-out provision. That's it, they say.
The more conservative members of the caucus won't budge either. They agreed Saturday to allow the debate to begin, but effectively killed the opt-out idea - Reid's attempt at compromise.
Right now, there is no public option plan that could garner 60 votes. A public plan "trigger" if private insurers fall short could come close - saying, losing Sanders but picking up Olympia Snowe (R-Maine) - but there's no guarantee it would fly in the House.
Enter "The Hammer," an idea from Sen. Tom Carper (D-Del.) States that lack affordable choices would be required to offer a national insurance program that wouldn't be government-financed or government-run.
Carper had already attempted a compromise with Sen. Chuck Schumer (D-N.Y.) - and came up empty-handed. Brown and others sound prepared to walk away if Democrats can't work this out.
"Four members of the Senate aren't going to tell the other 55 what to do on these issues," Brown said of the public option.
No room for abortion compromise
It's one of the most emotionally charged issues in the debate, with anti-abortion activists insisting that health reform cannot expand federal funding for abortions.
But when it comes down to whether Democrats can accomplish health reform this year, it becomes a vote-counting problem - how many anti-abortion Democrats would walk away from a bill they don't like?
Reid can't afford a single defection - and already, Sen. Ben Nelson (D-Neb.), who opposes abortion rights, says weak language could be enough to oppose final passage.
House Speaker Nancy Pelosi could lose at least 10 Democrats if the Senate tried to water down the tough anti-abortion language in her bill.
At the same time, about 40 Democrats in the House say they can't vote for a bill that goes as far as the House bill does now, with the so-called Stupak amendment that prevents a public insurance plan from offering elective abortions.
The big player here isn't Reid or Pelosi but the Catholic Church, which helped get the Stupak amendment into the House bill. Any abortion language may have to win the backing of the church for members to sign on - and the church is sticking by a tough anti-abortion stance that angers many liberals.
Is there a middle ground? That's not at all clear.
Millionaires vs. Cadillacs
Republicans pounded one big talking point all day Saturday: health reform raises taxes. And it's true, the plans would.
What's worse for Democrats is that the House and Senate have starkly different visions of how to pay for reform. And the House hates the Senate tax, and the Senate hates the House tax.
Not surprisingly, politics are at play. The House went with a populist soak-the-rich tax on "millionaires" to pay for almost half the near-trillion dollar price-tag in its bill. And bowing to pressure from powerful union backers, Democrats steered clear of any tax on the so-called "Cadillac" plans - high-cost policies that many unions have negotiated for their workers over the years.
Reid relies heavily on taxing the Cadillac plans - but won't touch a millionaires tax, which was never debated in the Senate.
Is there any give? Reid signaled he might be inclined to get a little closer to the House by saying he'd bump up the Medicare tax on high-earners. The "botax" on cosmetic surgery also seems aimed at the wealthy but only raises $5 billion. This will be one of the hottest debates when the House and Senate try to merge their bills.
Scaring seniors: Medicare cuts, higher premiums?
Republicans also tried to stir this sleeping giant Saturday, pounding Democrats for big cuts in Medicare under the Senate plan - at least $300 billion worth.
"So here we are telling the American people that we're going to fix health care in America and the way we're going to pay for the massive government takeover of health care is through cuts in Medicare?" said Sen. John McCain (R-Ariz.)
The reductions will almost certainly cut some seniors' benefits, a fact that has become a central opposition argument echoed from Capitol Hill to K Street. The cuts are likely to bump more than half the seniors currently enrolled in the popular Medicare Advantage program.
Democrats are keenly aware of the danger in a senior revolt and note that AARP, the nation's largest seniors' lobby, would not have endorsed reforms if they hurt seniors.
But there's another danger to Democrats lurking in the bill - dissent toward the White House deal with the drug-makers. Many Democrats feel PhRMA got off easy by only having to kick in $80 billion in cuts toward health reform. Some liberal Democrats want to change the deal's terms and force the industry to sell drugs to the federal government at a discount.
PhRMA insists that would bump up the seniors' Medicare prescription drug premiums by 20 percent. If the Senate includes the rebates, industry officials privately say that they'll consider running ads slamming senators for voting to increase seniors' drug costs.
"PhRMA would have to let people know the truth," said a senior pharmaceutical lobbyist. "I don't know why they would want to increase premiums."
Health fix fuels deficit worries
When Obama campaigned on enacting reform, he pledged to cut premiums, reduce the spiraling growth in medical costs and not add a dollar to the federal deficit.
It's not completely clear that he'll be able to accomplish any of those goals, and the public is catching on.
That's bad news indeed for Obama's efforts, especially at a time when voters already are giving him low marks for the sputtering economy and the 10 percent unemployment rate.
A Quinnipiac poll found that only 19 percent of voters believed Obama's pledge that health reform wouldn't boost the deficit in the next 10 years.
Doug Elmendorf, the Congressional Budget Office director, found that neither the House bill nor the Senate bill would add to the federal deficit. But federal spending for health care would go up under both bills in the next decade -- as much as $598 billion under the House bill over 10 years, roughly $85 billion in the Senate Finance Committee bill, the CBO said.
No one has been able to guarantee premiums won't rise. And there are serious questions about whether the bills go far enough to rein in costs.
That may explain why Democrats are now talking more about how the bills will expand coverage than about whether they will lower premiums for most families. And that's a big problem for the president and his congressional allies, given how uneasy independent voters and moderate Democrats feel about health care at this point.
In the absence of CBO data on premiums, expect Republicans to keep exploiting this weakness.
Chris Frates contributed to this story.