Judging from the scene on the House floor Saturday night, it was plainly obvious that something pivotal had just happened. The chamber’s passage of the Affordable Health Choices Act was the most substantial health reform passed since Medicare in 1965, and the bill’s razor-thin margin of victory, 220-215, gave a clear picture of just how close it was to failing. House Democrats filled the aisles to cheer the closing seconds of voting. Speaking to reporters afterward, Nancy Pelosi looked visibly moved. She said she “felt great,” and you could spot a tear running down her cheek.

Pelosi may want to relish her victory, but her time to do so is limited. Following the House passage, Sen. Lindsey Graham declared that the bill would be “dead in the water” in the Senate. Hyperbole, perhaps, but his notion is right. Pelosi’s finely curated piece of legislation will receive a much colder reception on the other side of the Capitol. At best, it’s in for substantial compromise. More likely, though, is a substantive overhaul.

The biggest sticking point in the Senate will remain the inclusion of a public option. To pass her bill, Pelosi had to compromise on a diluted version of one that would loosely compete with private insurance companies, but not much. As of now, only about 57 senators support any form of a public option, and several Republicans and at least one Democrat have vowed publicly to filibuster any bill that includes one. Majority Leader Harry Reid needs 60 votes to force cloture and end debate, and no matter how he has shuffled the cards, he still hasn’t been able to pick up the extras he needs.

Another issue at risk of extinction in the Senate version involves drug manufacturers. The House version, for the first time ever, allows a program like Medicare to negotiate drug prices, keeping manufacturers honest and competitive. But as Slate points out, that conflicts with a deal both Senate leaders and the White House struck with pharmaceutical makers in exchange for their support for broader reform. The Senate Finance Committee's bill currently includes this provision, and with Big Pharma’s support hanging in the balance, it’ll probably stay in.

And then there's the price tag. The House version costs about $1.2 trillion over a decade, while the Senate version costs rings in hundreds of billions less (about $900 billion). Examining line by line what should be kept will be a tedious task, but what Senate staffers could end up cutting are costly components, like one in the House bill requiring employers to provide coverage to employees, and enforcing it. Current Senate drafts include no such mandate.

Those changes might warrant gloom for Pelosi’s staffers, many of whom worked around the clock last week. But one potential upside might be the provision prohibiting abortion funding that was tacked on to the House version late in the game. Pelosi had to compromise on abortion to win over enough pro-life Democrats for the final vote. In the Senate, though, it might not be necessary. The main disagreements among senators are economic-based questions, like how the government will pay for the program, and whether it should meddle in the market. But abortion is more a social issue than a fiscal one, and Democrats like Joe Lieberman and moderate Republicans like Susan Collins and Olympia Snowe are much more likely to support funding for abortion regardless of how weak or strong the final health reform bill turns out to be. Republicans in the House sensed this could be a problem. Over the course of several hours of debate, House Minority Leader John Boehner asked every Democratic committee chairman to “guarantee” that the abortion exemption would remain in the bill compromised by the House-Senate conference committee. None of them would.

As the Senate prepares to begin its debate in the coming weeks, it’s worth remembering why the Senate has been at the center of attention on health reform. The House has too many moving parts to provide a compelling horse race, but the Senate calculus is dramatically simple—60 votes, no less. The fact that the Senate is second to take up the issue also makes its version more definitive, ensuring added contention from industries and interest groups making 11th-hour efforts. Pelosi may have moved the bill closer to the president’s desk, but what happens between here and there may quickly turn her tears of joy into ones of disappointment.