2020 Democratic presidential candidates John Hickenlooper, John Delaney and Steve Bullock appear on stage during the Democratic presidential debate in Detroit on Tuesday. (Anthony Lanzilote/Bloomberg News)

It has been building since the first primary debate in Miami ended, and on Tuesday night, it broke out into the open, a full-scale ideological brawl about the direction of the Democratic Party and what it will take to defeat President Trump in 2020.

In Miami, the presidential candidates collectively advocated for policies that highlighted the party’s dramatic shift to the left, including government-run health care that would eliminate private insurance, the decriminalization of the southern border and health care for undocumented immigrants. On Tuesday, the moderates in the field pushed back.

The fault line that was exposed between left and center now fully defines the Democratic nomination contest and will continue to do so as the candidates move through the summer and fall and head toward the primaries and caucuses early next year. It ultimately will be up to Democratic voters to resolve the differences, but the choices have become clearer and will now become harder to paper over.

The targets Tuesday were two of the most liberal and leading candidates in the field, Sens. Elizabeth Warren (Mass.) and Bernie Sanders (Vt.), who were accused of embracing “free-everything . . . fairy tale” policies and making “impossible promises” that could compromise the party’s chances of winning back voters they lost in 2016 — a loss that cost them the White House.

Warren and Sanders more than stood their ground during two spirited hours of sharp and passionate exchanges. At one point, a frustrated Warren shot back to one of her principal antagonists, former congressman John Delaney (Md.), “I don’t understand why anybody goes to all the trouble of running for president of the United States just to talk about what we really can’t do and shouldn’t fight for. . . . I’m ready to get in this fight. I’m ready to win this fight.”

Ahead of Tuesday’s debate, there were questions about whether either a rising Warren or a slipping Sanders, competing for many of the same voters, would go at each other. Instead, they were forced into solidarity, at least for the night, after they came under assault during the other candidates’ opening statements.

It began with Montana Gov. Steve Bullock, who did not qualify for the first debate in Miami and was under pressure to make his mark quickly. “Watching that last debate,” he said, “folks seemed more concerned about scoring points or outdoing each other with wish-list economics than making sure Americans know we hear their voices and will help their lives.”

Moments later, Delaney, who has been in the race longer than anyone but has little to show for it, was even more pointed. “Folks, we have a choice,” he said. “We can go down the road that Senator Sanders and Senator Warren want to take us, which is with bad policies like Medicare-for-all, free everything and impossible promises that will turn off independent voters and get Trump reelected.”

But they were not the only ones to challenge Warren and Sanders. Critiques came from former Colorado governor John Hickenlooper, Sen. Amy Klobuchar (Minn.) and Rep. Tim Ryan (Ohio). Pete Buttigieg, the mayor of South Bend, Ind., sought to walk a careful line between the two wings, as did former congressman Beto O’Rourke (Tex.). Author Marianne Williamson, animated as much as anyone, found ways to go in both directions.

Health care dominated the early portion of the debate, and it turned into the most pointed discussion of whether the party should embrace Medicare-for-all and the elimination of private health insurance or pursue a path that would build on the Affordable Care Act by adding a public option or another plan to provide universal coverage. It was, as Hickenlooper cast it, a question of “an evolution, not a revolution.”

Sanders, who was far more energetic Tuesday than he was in Miami, has said repeatedly that his plan would require raising taxes on the middle class, although he added that, in the end, Americans would pay less overall for their health care than they are now paying.

CNN moderator Jake Tapper pressed Warren and others who favor Medicare-for-all to answer whether they would do the same. Warren hedged. “So giant corporations and billionaires are going to pay more,” she said. “Middle-class families are going to pay less out of pocket for their health care.”

Ryan said that Medicare-for-all would mean that union workers, who have negotiated for good health-care coverage sometimes at the expense of wage increases, would lose those benefits. Sanders insisted that his plan would allow for benefits as good or better than what those workers now have.

“You don’t know that, Bernie,” Ryan responded.

“I do know it,” Sanders replied. “I wrote the damn bill.”

The sparring that began Tuesday will no doubt be repeated on Wednesday, when former vice president Joe Biden, who has clashed with Sanders over health care, will be at center stage and looking to improve on a lackluster performance in Miami.

He will be next to Sen. Kamala D. Harris (Calif.), whose attack on Biden in Miami became the most memorable moment of the two June nights, and he could tangle with Sen. Cory Booker (N.J.), who has been aggressive in signaling his differences. But those battles will have to compete with the other questions left unresolved on Tuesday night.

Among the Democrats, there is clear sensitivity to how Trump’s reelection campaign will attempt to lump all the candidates together, painting the party’s eventual nominee with a broad brush no matter his or her positions. Sanders made reference to that issue, challenging the premise of some of the questions, calling one a “Republican talking point.”

“It is time to stop worrying about what the Republicans will say,” Buttigieg said at one point. “It’s true that if we embrace a far-left agenda, they’re going to say we’re a bunch of crazy socialists. If we embrace a conservative agenda, you know what they’re going to do? They’re going to say we’re a bunch of crazy socialists.”

But many Democrats do worry. Not just how the Republicans will attempt to brand their nominee and the party but the degree to which a historically liberal agenda would make winning states such as Michigan, Pennsylvania and Wisconsin far more difficult.

Hickenlooper said elements of the Green New Deal, a liberal climate change plan, would be “a disaster at the ballot box. You might as well FedEx the election to Donald Trump.”

Ahead of Tuesday’s debate, the candidates received unsolicited advice from some of the party’s current and former elected officials. The message: Don’t make Detroit a rerun of Miami.

Rahm Emanuel, a former mayor of Chicago and a veteran of the Obama and Clinton White House teams, put it bluntly in a publicly posted memo to candidates on Monday that criticized the candidates’ performances in Miami.

“Too often, you succumbed to chasing plaudits on Twitter, which closed the door on swing voters in Wisconsin, Michigan, Pennsylvania, and Ohio,” he wrote. “If you win the nomination in a way that forecloses a path to victory in the general election, we will lose, and your name will go down in infamy.”

The Trump campaign found Emanuel’s memo so appealing that its rapid response operation sent out an email Tuesday morning pointing to it. Beyond that, the president’s campaign took out ads in the Detroit News and the Detroit Free Press with a message that the Democrats would move the country dangerously to the left if their nominee wins in 2020. “They’re all the same,” the ad read, following by a subheadline that proclaimed: “Democrat plans to wreck America.”

Many Democratic strategists dismiss such charges, as the candidates did on Tuesday night. But several Democratic governors recently expressed their concerns to the New York Times, and on Tuesday, the Democratic Governors Association urged the contenders to follow the lead of the governors and other Democrats who won in 2018 by emphasizing “kitchen table” issues.

Tuesday’s debate brought to the surface the substantive differences and questions of political strategy that the Democrats now must resolve among themselves before they will be truly ready to take on the president in November 2020.