HEMPSTEAD, N.Y.—Before this week’s first presidential debate, it was common for Donald Trump’s television surrogates to predict it would echo the sole 1980 encounter between Jimmy Carter and Ronald Reagan.

It turned out, to borrow from another famous debate moment, Donald Trump was no Ronald Reagan.

On the surface, the analogy appeared reasonable. Like Hillary Clinton today, Carter in 1980 bet most of his chips on personally disqualifying Reagan. Carter painted his opponent as unqualified, ill-informed, extreme, and dangerous—an aging entertainer who might trigger a nuclear war through ignorance and belligerence.

For months, enough voters feared Carter might be right to keep him close in the polls, despite enormous dissatisfaction with his job performance. But when Reagan in the debate presented himself as composed, reasonable, and genial (swatting away even accurate Carter recitations of his most outrageous earlier statements with a jaunty “There you go again”) the doubts softened, Carter’s support crumbled, and the Gipper rolled to a landslide.

Trump’s television chorus confidently predicted a repeat. Trump, they promised, would show himself as a man that Americans could easily envision representing them to the world, managing the military, and breaking gridlock in Congress. More than a few Democrats feared they might be right. The Clinton campaign had portrayed Trump so ominously that he might have discredited the picture if he had just made it through 90 minutes seeming polite and reasonable.