Ten Democratic candidates take the stage befor the start of their presidential debate on Thursday in Houston. (David J. Phillip/AP)

One of the presumptions going into the third Democratic presidential debate was that finally having the three leading contenders on the same stage would be a long-awaited inflection point, narrowing the field and defining the choice for a party desperate to find its champion to take on President Trump in 2020.

The leading trio — former vice president Joe Biden and Sens. Elizabeth Warren (Mass.) and Bernie Sanders (I-Vt.) — did well enough. Biden, in particular, was steadier and more aggressive than during his two previous debate performances, which may quiet some of the doubts about him. And Warren, though she did not land any memorable punches, did what she does best: staying focused and driving her own message.

But it was the candidates further down in the polls who showed more of what it will take to defeat Trump. That is why Democrats should not be impatient to usher these competitors off the stage anytime soon.

Nowhere was that clearer than in the lengthy exchange over health care, the issue that has, thus far, represented the central divide within the Democratic Party.

By now, anyone who has been paying attention is familiar with the basic outlines of the argument between the Sanders-Warren faction that pushes a single-payer system, and the Biden wing, which advocates a more incremental approach that would build on the foundation of the Affordable Care Act. Just as familiar is the Republican reaction each time the subject is raised, which is to raise the red flag of socialism.

On Thursday, it took Sen. Kamala D. Harris (Calif.) to elevate that discussion and to remind Americans what is really at stake for health care in next year’s election.

“Everybody on this stage, I do believe, is well-intentioned and wants that all Americans have coverage and recognizes that right now 30 million Americans don’t have coverage. But at least five people have talked, some repeatedly, on this subject, and not once have we talked about Donald Trump,” Harris noted before ticking off the efforts that the administration has made and is continuing to make to gut the health-care law.

“If we don’t get Donald Trump out of office,” Harris added, “he’s going to get rid of all of it.”

On the question of racial justice, Sen. Cory Booker (N.J.) was the one who brought both eloquence and critical perspective with his reminder that the problems with race in this country did not begin with Trump and will not end when he is gone.

“We know Donald Trump’s a racist, but there is no red badge of courage for calling him that,” Booker said, adding, “this is not just an issue that started yesterday. It’s not just an issue that we hear a president that can’t condemn white supremacy.”

The evening’s other fine moments included the passion with which former Texas congressman Beto O’Rourke spoke of the mass shooting in his hometown of El Paso, and his fearlessness in advocating his controversial proposal that owners of assault-type weapons be forced to sell them to the government; Sen. Amy Klobuchar’s (Minn.) no-nonsense pitch for unity; and South Bend, Ind., Mayor Pete Buttigieg’s moving account of deciding to risk his political future by coming out as gay. Even former HUD secretary Julián Castro’s botched dig at Biden’s age was instructive in its own way.

Nearly all the candidates are better and more compelling than they were when the race began, and there are still more than 140 days to go before the first ballots will be counted in Iowa.

What is likely to happen between now and then? One or more of the competitors now thought of as long shots are likely to have a breakout moment, while someone in the front of the pack may stumble. Potential surprises could reshape the electability argument. (It is worth remembering that around this point during the 2004 race, the flash-in-the-pan leader of the 10-person Democratic field was Wesley Clark , a retired Army general. John F. Kerry, the ultimate nominee, was struggling to hang on to third place in the polls.)

Done right, a vigorous primary can be a healthy thing, as the long battle between Barack Obama and Hillary Clinton proved in 2008. And if candidates cannot hold up under criticism from those who agree with them on most things, it is difficult to have much confidence in how well they would weather the kind of attacks they would be certain to receive from a president who is restrained by neither facts nor norms.

So take a breath, Democrats. Don’t be impatient for this race for the nomination to head into its final stretch just yet. Whoever comes in first at the finish line is likely to be a sharper and better candidate for having been tested by the ones who bring up the rear.