For weeks, Democrats have argued over how best to take advantage of their perhaps fleeting control of both chambers of Congress. Progressives want to use “reconciliation” — a parliamentary maneuver that permits taxing and spending bills to pass with a bare majority, instead of the Senate’s usual 60-vote requirement — to push through a far-ranging social spending bill. Moderates have balked at spending $3.5 trillion over 10 years, much of that on programs that could be slimmed down.

Now it is crunch time. President Biden held marathon talks Wednesday to bridge the differences, but the two sides remain far apart. All of them support a separate, $1.5 trillion infrastructure bill that passed the Senate with bipartisan support. But progressives say they will vote it down on Monday unless moderates back their vision of a reconciliation package. Moderates want the infrastructure bill to pass whether or not there is agreement on reconciliation; the country needs it now, they say, and Democrats could show they can deliver. Progressives worry they would then lose leverage in future reconciliation negotiations.

The upshot is that they could end up delivering nothing: no infrastructure bill, no larger social spending bill. Democrats would squander a rare opportunity to address generational problems such as climate change, health-care access and wealth inequality. They would keep Mr. Biden from delivering on his promise to make Washington work again. Failure could empower Republicans, for whom embracing the poisonous lie that Mr. Biden lost the 2020 election is becoming a requirement for holding office, with potentially dire consequences for U.S. democracy.

There is a way through: substantive compromise. Moderates have complained about the reconciliation bill without making a counteroffer. They must provide an alternative. Mr. Biden appears to have persuaded them to do so. Progressives must then be open to the moderates’ offer. Negotiations should center not on how much should be spent, as they have so far, but on which programs deserve funding and on how to design them to reach those with genuine need.

Expanding the child tax credit would halve the child poverty rate. Ensuring access to pre-kindergarten education would relieve pressure on working families. Pumping up Pell grants would enable low-income people to afford college. Long-needed policies on global warming would help rescue the planet from climate disaster. One way for moderates and progressives to bridge their divides is to means-test new social spending, directing it only to those who need it. Rich seniors do not need new Medicare benefits. Similarly, reinforcing the Affordable Care Act would make a much larger difference to low-income people than lowering the Medicare eligibility age. Reasonable tax hikes on the wealthy and on corporations could then cover the cost.

If Democrats agree on a compromise framework before Monday, progressives could vote for the infrastructure bill with more comfort that a reconciliation package is on track. Alternatively, if negotiations are not quite complete, moderates could allow House leaders to delay a vote on the infrastructure bill in the knowledge that good-faith negotiations are underway.

This is not easy. Wide philosophical chasms divide Democrats: Should social programs be means-tested or universal; how much should the government direct the economy; how high should taxes be raised; how fast should change occur? Mr. Biden and the Democrats must show that people with deep, principled disagreements can shake hands on a plan that none think is perfect — but that would make tangible, sizable progress for the nation.