(Bloomberg Opinion) -- When presidential fields form, the relatively small number of people who are paying attention at the beginning evaluate the candidates based on how well-known they are, their strengths of support from important constituencies, the distinctive features of their biographies, how much money they have raised, and so on. But there’s also the question of their political talents, and it’s one that can only be answered in the course of a presidential campaign.During the 2016 primary season, a lot of Republicans thought Senator Marco Rubio of Florida had great promise -- but they also had doubts about him. Watching him stick robotically to talking points under a sustained assault from Chris Christie in a debate that February tended to confirm those doubts, and helped to eliminate him from the race.Now it’s Democratic presidential candidates who are being tested. Senator Kamala Harris, as a black female senator from the biggest state in our country, was always likely to be a strong contender. In the second of the first two Democratic debates, though, she also stood out as the coolest and toughest professional on the stage. Joe Biden has also been a top candidate, on paper, having served as vice president in an administration Democratic voters dearly miss. But his debate performance, while not deadly, is likely to deepen those voters’ worries that he is not up to the job of beating President Donald Trump in 2020.The exchange between Harris and Biden was the most important moment from either debate. That’s mostly because it was one of the few arguments between two of the top five candidates -- those two, Mayor Pete Buttigieg, and Senators Bernie Sanders and Elizabeth Warren -- and the only truly testy one.Harris hit Biden for his “hurtful” remarks about segregationists and his long-ago opposition to busing. Biden came back by claiming that Harris had misrepresented his remarks; taking a swipe at Harris for having been a prosecutor rather than a public defender, as he was; defending his record on busing as a defense of local control; and talking about his civil-rights record more generally, including his time in the Obama administration. And then, he announced that he had exceeded his time and would stop talking -- a striking decision by a famously wordy politician.There is something otherworldly about that exchange. There is no great public demand for the return of busing for racial balance in schools, and Harris isn’t really proposing it. But she showed that she is willing to hit Biden hard without blinking, and he showed that he doesn’t have a forceful answer to one of the main charges against him. (Whatever the merits of local control, it is hard to make it palatable to a Democratic audience, especially in the context of civil rights.)Biden has always rambled, but that tendency adds to the liability of his age. Eric Swalwell, a California congressman on the stage for whatever reason, probably did himself no favors by unsubtly raising Biden’s age. But he’s not the only Democrat who is thinking about it.The other top-five candidates faded into the background. Sanders is suffering this time around because a lot of people, possibly including the senator himself, misunderstood his strong showing in 2016. They thought it was about him and his promise of socialist revolution. Beyond a hard core, it was about his idealism in contrast to Hillary Clinton’s cynicism. Without Clinton in the race, his true base turns out to be smaller.Warren has been gaining at his expense, but has to show that she can recover from a terrible 2018, when her DNA-testing stunt backfired so astonishingly she had to apologize to the entire Cherokee Nation, and made Democratic insiders worry about her electability. Buttigieg rose rapidly and proved himself to be polished and thoughtful, but did not do much to defend himself -- even from Swalwell’s criticism of his handling of a police shooting in his city of South Bend, Indiana.Even for veterans of races for other offices, there is nothing like the pressure of presidential politics. There’s also no substitute for seeing how the candidates do under it. For a brief moment, it may have looked as though Biden’s popular support would see him through to the nomination while his opponents raged online. The first debates make it clearer than ever that he’s going to have to fight for it -- if he still has it in him.The debates may, however, have opened one path for Biden. Harris, Sanders and Warren have all come out for “Medicare for All” legislation that outlaws almost all private health insurance – throwing scores of millions of Americans out of plans they like. (Harris emphasizes, however, that private coverage would still be available for cosmetic surgery and the like.) It has mostly been lower-tier Democrats who have made the case against disrupting Americans’ health coverage this way. The polls already show that most voters don’t want to abolish private insurance. Many senior citizens will be livid to learn that Medicare for All also abolishes Medigap and Medicare Advantage plans.Biden could use this issue to say his rivals are falling for ideological fads that endanger the Democrats’ chances in 2020. That critique would have the merit of being true. Which means that Biden’s problems are also his party’s.To contact the author of this story: Ramesh Ponnuru at rponnuru@bloomberg.netTo contact the editor responsible for this story: Tobin Harshaw at tharshaw@bloomberg.netThis column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.Ramesh Ponnuru is a Bloomberg Opinion columnist. He is a senior editor at National Review, visiting fellow at the American Enterprise Institute and contributor to CBS News.For more articles like this, please visit us at bloomberg.com/opinion©2019 Bloomberg L.P.