Biden Won Over Mitch McConnell. Now He Needs AOC.

The moment the gavel came down on the Senate’s passage of the Infrastructure Investment and Jobs Act today, the bipartisan bill began a new life as a 2,700-page legislative hostage.

For President Joe Biden, securing the votes of well over a dozen Senate Republicans on one of his top legislative priorities is an achievement that many doubted was possible. Whether that infrastructure bill ever makes it to his desk, however, now depends on the outcome of a negotiation that may prove nearly as tricky—unifying the sniping factions of the Democratic Party.

Under pressure from progressives, House Speaker Nancy Pelosi has vowed to shelve the $1.2 trillion infrastructure bill until Senate Democrats, using a budget process that circumvents a Republican filibuster, deliver on their commitment to pass a far more ambitious, $3.5 trillion package to expand social programs and tackle climate change. Because Republicans are sure to oppose that second bill unanimously, Democrats will need the votes of all 50 of their senators (plus a tiebreaker from Vice President Kamala Harris) and nearly every member of their razor-thin House majority. That means finding a compromise that can win the backing of moderate Senators Joe Manchin of West Virginia and Kyrsten Sinema of Arizona as well as leftists, including Senator Bernie Sanders of Vermont and Representative Alexandria Ocasio-Cortez of New York.

[Read: The Republicans have already given Biden what he needs]

Senate Democrats are expected to move quickly to advance the blueprint for that bill in the coming days, before leaving Washington, D.C., for the rest of August. But the actual legislation—the nitty-gritty policy details translated into thousands of pages of text—likely won’t be ready before the fall; until then, the freshly passed infrastructure bill will gather dust. For the moment, Democrats have only superficial unity on their budget. The core of the plan is a collection of long-sought progressive priorities, including paid family leave, universal pre-kindergarten, free community college, new investments in green technology, and a modest expansion of Medicare and other federal health-care programs. But big questions remain unanswered: How aggressively will the bill combat climate change? How long will these new programs—as well as the previously enacted expanded child tax credit—be funded for? Will Democrats be able to include immigration provisions to protect Dreamers and other undocumented people? Whose taxes will go up to pay for it all, and by how much?

Already, Manchin, who represents conservative coal country, has said he’s “disturbed” by elements of the proposals to fight climate change. Meanwhile, Sinema has said flatly that she won’t support a final package that costs $3.5 trillion, leading to a tweeted rejoinder from Ocasio-Cortez and a none-too-subtle threat to take down the bipartisan bill that the senator so carefully helped to negotiate. Moderates in the House are demanding that Pelosi bring up the bipartisan bill immediately and warning her not to take their support for a second package for granted. Apart from the ideological divide, senior Democrats like Representative Peter DeFazio of Oregon, the ornery chair of the transportation and infrastructure committee, are complaining that the bipartisan bill is insufficient and an encroachment on their policy turf. All the bickering highlights how risky Biden’s gamble still is: If Sinema and other moderates tank the $3.5 trillion bill in the Senate, Democrats could vote down the bipartisan bill in the House, and the president could be left with nothing.

For Biden, the good news is that he is a president particularly well suited to landing on the sweet spot for his party. If he had a singular talent over the course of nearly half a century in elected office, it was in finding the political center—not necessarily of the country as a whole, but of the Democratic Party. Biden has drifted with Democrats over the years, usually to the left but rarely with a lurch, so that wherever the party’s mainstream is, that’s where he’s swimming. He did so capably enough to win the nomination against younger, more progressive candidates. But he’s also proved that skill as president, by winning passage of a $1.9 trillion COVID-19 relief package on party-line votes this spring.

The president also benefits from a unity of mission among Democrats. Until recently, whether Republicans actually wanted an infrastructure bill signed into law was a question mark, but there seems to be little doubt—at least for the moment—that virtually the entire Democratic Party in Congress is invested in the success of the larger reconciliation package. The party knows it’s an underdog to hold its majorities in the midterm elections next year, and a cascading failure in which both the budget package and the bipartisan infrastructure bill fall short would be devastating to that effort. The desperation to avoid that kind of embarrassment is partly what drove Democrats to push through the Affordable Care Act in 2010 after the loss of a crucial Senate seat in Massachusetts. Their legislative proposals now are more popular with the public.

In the days after the initial announcement of a bipartisan infrastructure accord in the Senate, Biden, to keep Republicans from bolting, had to issue a statement saying that passage of the deal was not linked to the passage of the broader Democratic agenda. The president’s assurance was written, not recorded, or you might have seen him winking. Now that he has the Republican votes in hand, Biden can dispense with such niceties. The infrastructure bill may be a hostage, but the GOP can no longer touch it, nor can it stop the much more transformative package Biden is seeking. The president needs only his own party—a far friendlier negotiating partner—to deliver them both.