It’s crunchtime for President Biden’s legislative agenda, and yet there’s so much that remains unknown.

We don’t know how many Republicans will vote for the bipartisan infrastructure bill. We don’t know how House Speaker Nancy Pelosi (D-Calif.) and Senate Majority Leader Charles E. Schumer (D-N.Y.) will steer that bill and the $3.5 trillion social spending package across the finish line. And most important, we don’t really know why moderate Democrats remain open to tanking the whole thing.

After all, moderates will insist they back both bills — at least in theory. Sen. Joe Manchin III (D-W.Va.) claims to support the reconciliation bill even as he urges a “strategic pause” on it. “I support reconciliation,” said moderate Rep. Josh Gottheimer (D-N.J.) on CNN’s “State of the Union” on Sunday. “And so do my colleagues. We meet all the time. And let me tell you, we all agree that we need a reconciliation package.”

And moderates have spoken up about specific pieces they object to. Sen. Kyrsten Sinema (D-Ariz.) and a handful of House Democrats oppose letting Medicare negotiate prescription drug prices, which would save more than $500 billion over 10 years. (Incidentally, three of the Democrats opposing the idea have received $1.6 million in donations from the pharmaceutical industry.) The New York Times also reports that Sinema “has privately told colleagues she will not accept any corporate or income tax rate increases,” while Manchin remains opposed to the current plan’s proposed corporate tax rate. Gottheimer wants to repeal the cap on state and local tax deductions, which would help many of his wealthiest constituents.

But while it’s one thing to hold these views, it’s another to argue that these objections should derail the broader package that these lawmakers (and a majority of Americans) support. If Sinema swallows the tax increases she doesn’t like, she gets the climate measures that she prioritizes. If Manchin agrees to not means-test the expanded child tax credit, he gets billions in infrastructure spending.

That’s called compromise — just as progressives have already compromised by reducing their ask by trillions. The final number may well end up higher than these moderates would like. But, as President Biden noted during a Friday news conference, when skeptics of the bill “go through their priorities, it adds up to a number higher than they said they were for.”

No doubt moderates don’t like being pressured to vote for things they don’t completely support. No one ever does. But opportunities such as this don’t happen often. As Sen. Cory Booker (D-N.J.) said on NBC’s “Meet the Press” on Sunday, “I have been looked in the eye by people in this town and they’ve meant it sincerely. There’s honor in them saying it. ‘Hey, Cory, we’re not going to get this in such and such a bill, but we’ll get to it.’ Well, sometimes that ‘get to it’ turns out to be a year, the next Congress, or what have you. I’m sorry, when you have the leverage, you use the leverage, as long as it’s not about ego or partisanship.”

It’s not every day that Democrats hold both houses of Congress and the White House. In the past 50 years, most presidents who have had one-party control of Washington have lost it after two years, from Bill Clinton to Barack Obama to Donald Trump.

Because a Republican-controlled House or Senate will almost certainly never give Biden a win, history suggests that this Congress is the Democrats’ last shot at passing major legislation until the next Democratic president. If moderates want to kill these bills, then they need to explain not just what they object to, but why those objections justify derailing the last, best chance at big change for years.