Joe Biden and Bernie Sanders do not want it on their consciences that they helped reelect President Trump. The future of the country really does depend on how both of them act over the next few weeks.

They should realize four things. First, if they don’t work together, and fast, Trump really could win reelection. If the urgency of putting an end to a genuinely dangerous presidency was not clear before his bungling of the coronavirus crisis, it should be now. Second, they need each other to achieve their respective core goals, both in the short term and in the long run. Third, there is a history of the left working with a governing center to achieve substantial social reform. Finally, neither of them has a monopoly on wisdom. Each must pay attention to the other’s central concerns.

What will be asked of them is not easy: Sanders has to acknowledge, quickly, that there is no way he is going to win the Democratic nomination he has spent five years fighting for. Biden has to reach out to a candidate who has regularly attacked him as a tool of the establishment and to a constituency of young voters who think he’s over the hill.

If they can’t do all these things, they will strengthen Trump in November. The difficult truth is staring them in the face: They need each other to win.

Democrats showed in 2018 the good things that can happen when their progressive and moderate wings work together. The party grabbed control of the House with a popular-vote majority of nearly 10 million. That’s enough to retake the White House.

If the vote for president matched the popular vote for House candidates in every state, according to calculations by my Brookings Institution colleague William Frey, the Democratic nominee would get 293 electoral votes. That means victory, but it’s only 23 more than the 270 required to win the presidency, which is not much of a cushion. Biden thus badly needs the constituencies Sanders has mobilized, particularly Americans under 35. Biden will be sunk if they vote for third-party candidates or stay home.

In 2018, progressives realized they had an interest in victories by many politicians around the country with more moderate views than their own. Moderates, in turn, grasped the turnout and activism that progressives could provide. Both sides also understood that it’s foolish to argue about whether elections depend solely on either mobilizing voters or persuading them. If all the wings of the Democratic Party don’t mobilize, Biden will lose. But especially in swing states — Pennsylvania, Michigan, Wisconsin and Arizona are at the top of the list — Biden will also lose if he doesn’t persuade at least some of Trump’s 2016 voters to switch sides and many moderate independents and Republicans to come his way.

This is why Biden needs Sanders, but also why Sanders needs to give Biden some room to do what he needs to do to win. Accepting defeat might make Sanders grumpy, but he can move forward with pride — and that is what he seemed to do when he signaled a de-escalation of the campaign Wednesday. He acknowledged he was “losing” but said his forces had “won the ideological debate.”

He was onto something. He has fundamentally altered the nature of the conversation in the Democratic Party. Together, he and Elizabeth Warren have moved even the most moderate Democrats to the left. His proposals for Medicare-for-all, free college and bold action on climate change, and his warnings about the dangers of concentrated corporate power, set the terms of this year’s debate. Candidates were judged by how close or far they were from his core objectives. That’s a big deal.

Moreover, Sanders’s movement is now a permanent force in the Democratic Party, much like organized labor was in the old days, or the religious right is now in the GOP. Democrats will not be able to ignore the left wing that Sanders has brought to life. He made democratic socialism a popular idea among young Americans. Unlike their elders, they associate socialism not with the Soviet Union but with a decent, democratic society that pays attention to the needs of the powerless, the marginalized and the working class. A 2018 survey showed that 58 percent of adults under 30 identified socialism as a system that “provides citizens with health insurance, retirement support, and access to free higher education”; only 38 percent saw it as government control of “key parts of the economy, such as utilities, transportation and communications industries.”

But for all his achievements, it’s clear now that Sanders speaks for only about a third of Democrats. (So far, he has received just over 30 percent of the popular vote in the Democratic primaries.) His task is to accept his role as a source of pressure for far-reaching reform inside the Democratic coalition.

There’s a role model for him: Sidney Hillman, the strategic mastermind of the Congress of Industrial Organizations, the left wing of the labor movement of the 1930s and 1940s. The CIO gave critical support to Franklin D. Roosevelt when he needed it against the party’s right wing, but it also pushed FDR in a more progressive direction.

Sanders and his movement should embrace the idea that they need a sympathetic moderate liberal — his name now is Joe Biden — in power to achieve anything they have in mind. They have a right to ask Biden for some reassurances, but they should not waste a lot of time on foolish platform fights or demands for symbolic gestures. What matters is the policy agenda Biden pursues after he is elected.

The good news for Biden is that there are many issues on which outreach to Sanders’s supporters could also help the likely Democratic nominee with the broader electorate, too. A commitment to increasing the power of workers in an economy that has seen unions weaken sits well with both Sanders’s objectives and Biden’s Scranton, Pa., working-class roots. More vocal and aggressive moves on climate change would help reassure younger voters. Strong pledges to limit the power of big money in politics would reinforce the message House Democrats sent with their comprehensive elections and ethics bill, H.R. 1. Biden could also take some steps closer to Sanders on universal access to public universities and job training. And while Biden would be ill-advised to change his position on health care, he could shift from bashing Medicare-for-all to giving Sanders credit for moving the broader health-care argument in a more progressive direction.

Finally, Biden’s pledge to work with Republicans is too central to his persona for him to abandon it. But he needs to do far more to show that he recognizes the growing power of right-wing radicalism in the GOP and its habit of obstructionism.

More broadly, Biden needs to make very clear that he accepts the Sanders movement’s critique of Democratic politicians who gave in too readily to the right-wing political consensus that Ronald Reagan established after 1980. Democrats have spent too much time negotiating with themselves and offering preemptive concessions that rarely won them Republican support. They built the Affordable Care Act around Republican ideas, made one change after another at the GOP’s request — and won not a single Republican vote on final passage. When they are in power, Democrats are terribly sensitive to Republican criticisms of deficit spending, and they work to bring budgets under control; then they look on when Republicans win power and let the red ink flow freely to finance tax cuts for their wealthy supporters. Sanders’s backers are understandably tired of Democrats always looking over their right shoulders.

Biden also should reject the premise of another foolish argument: that Democrats must choose between restoration or transformation as the core goal of the next four years. Plainly, the country needs both. Restoring the norms and values that Trump has ripped apart is a precondition of progress. But that’s the point: What voters seek to restore is progress.

The next president needs to be mindful that Trump’s election was a sign of how much transformation the country needs. The growing economic inequalities of the past 40 years — with their racial, regional and individual components — helped Trump win and made it easier for him to divide the country into warring factions.

And here is what Biden and Sanders should acknowledge together: Democrats are a far more progressive party today than they were even in 2008, when Barack Obama was elected president. This is true both because of pressure from the inside — from Sanders and Warren, and from new left-of-center voices such as Reps. Alexandria Ocasio-Cortez (N.Y.) and Ayanna Pressley (Mass.) — and because nearly everyone has a less sanguine view of unregulated capitalism since the great crash of 2008.

Polling in the past few weeks suggests that 80 percent or more of Sanders’s supporters are already inclined to vote for Biden over Trump. If Biden reaches out, as he should, Sanders should reciprocate by dissociating himself more forcefully than he has from the most toxic among the Bernie Bros — even though he also has a right to remind Democrats that the vast majority of his backers are idealistic partisans for social justice who don’t spend their lives on Twitter. One Sanders organizer asked me in Iowa in January: “Why do the media judge us by the loudest of our supporters rather than the kindest of our supporters?” Sanders should also stop claiming that Biden’s victory was a product of dastardly moves by “the Democratic establishment.” In fact, Biden’s victory was built on a foundation of African American votes in South Carolina and elsewhere, and on the intense desire of millions of middle-class Democrats to beat Trump.

It takes a coalition to save a country, and coalitions are hard. They succeed when their members focus on what they share rather than what pulls them apart. Starting with Sunday’s debate, Biden and Sanders have to begin a process of reconciliation that will send a message to their respective supporters, and the rest of the nation, that change begins with getting Trump out of office — but it won’t end there.

Two men in their late 70s should be especially mindful of how history will judge them. If they get this wrong, nothing else they have done will matter.

Twitter: @EJDionne