Muhammad Ali was never the hardest puncher in boxing. Two things made him “the greatest”—and allowed him to defeat a lot of other fighters who could hit harder. One was his tremendous tactical intelligence in the ring. The other was that, unlike most of his opponents, Ali could inflict injury in retreat as well as while attacking.

Watching Hillary Clinton debate Bernie Sanders, I was reminded of Ali’s prowess. Clinton’s ability to think on her feet is formidable, and if anything only improves with pressure and experience. Whether the topic was healthcare or regulating finance or her ties to Wall Street or her vote on the Iraq War or her weathervane turns on TPP and the Keystone Pipeline, rare was the occasion when Clinton, facing either a potentially embarrassing question or a frontal Sanders attack, didn’t manage to call into question either the Vermonter’s character (charging him with engaging in “artful smears”), his judgment (on calling for the United States to move towards normalizing its relationship with Iran), or, most often, his grasp on reality.

In one way what we saw last night in New Hampshire was the debate we have all been waiting for—about the essential questions of war and peace, prosperity and equity. Most of all, it was a debate about the boundaries of political possibility. Clinton is a reformer, a liberal, a believer in experts and expertise, and a tough cop when it comes to enforcing conventional wisdom. Sanders is an idealist, and an ideologue, and a populist who really believes the game is rigged and doesn’t believe that shaking your finger at the referee and telling the players to “cut it out” will change that. Certainly either of them fit comfortably under the fuzzy blanket of “progressive.”

If the Republicans could be relied on to nominate a candidate who would be equally constrained by reality it would be a pleasure to see Clinton take, say, Jeb Bush apart in debate. But of course no such guarantees are possible—or even, at this point, likely.

And running for president is even harder than becoming heavyweight champion of the world. If Clinton is going to win in November, she needs to learn how to damage Sanders—or out-point him—without denigrating the goals she claims they share. She needs to defeat him without making his supporters come to hate her—a problem Ali never faced.

If Sanders is going to win more than just New Hampshire—especially if he’s going to have a realistic chance of winning in November—he’s going to have to become a much better counterpuncher, and to learn to think on his feet. He doesn’t need to attack Clinton’s character, but he does need to explain why her vision isn’t good enough—and what those “smart hedge-fund guys” she talks about think they’re getting when they contribute to her Super PAC. Each of them could learn an awful lot from the other.