One large, strategic question at the center of the presidential race: In order to beat President Obama, does Mitt Romney need to be an exceptional candidate or merely an acceptable one?

For much of the campaign, Romney and his team have operated according to the acceptability theory. Obama, in this view, is a vulnerable incumbent who will eventually be undone by a stagnant economy. With 54 percent of likely voters already agreeing that Obama does not deserve re-election, Romney (the argument goes) just needs to be a viable alternative — an imaginable president — in order to defeat an incumbent in the process of failing.

Romney’s convention speech was the triumph of this theory. It was designed for reassurance, not persuasion. It emphasized Romney’s admirable background and values. It was almost entirely devoid of policy arguments, unexpected outreach or effective attacks. It was unambitious for a reason — because acceptability seemed the safest policy.

But the approach didn’t work. Following the party conventions, it became clear that Obama has a hidden source of electoral buoyancy. Many Americans don’t fully blame him for economic conditions and seem resigned to a new normal of high unemployment and stagnant growth. It is a historical paradox of the first order — the candidate of hope made viable by diminished expectations. But Obama, it is now evident, will not fall by force of gravity.

This was the broader significance of Romney’s performance in the first debate. It was more than an excellent technical performance. It was an admission that acceptability is not a sufficient strategy. For the first time in the general election, Romney seemed to realize that the presidency will not be awarded by default — that defeating Obama will require exceptional skills, strategy and ambition. And all were there when Romney needed them.

Romney was on the offensive from first to last, dominating the tone, content and flow of the debate. This seemed more than aggressiveness; something approaching authority. Romney’s attacks were genially relentless.

Instead of merely criticizing Obamacare or the Dodd-Frank financial legislation, he dissected them. His fired statistics like shotgun pellets — 23 million unemployed, one in six in poverty, 50 percent of college graduates can’t find jobs. His critique was organized by a memorable theme — “trickle-down government.” (Obama’s apparent theme — a “new economic patriotism” — went entirely unexplained.)

Romney’s effective indictment of Obama’s record managed something difficult and important. It simultaneously steadied the confidence of Republicans in their own candidate while allowing Romney to adopt a more moderate, bipartisan tone on taxes, education and entitlements. This is politics successfully conducted at a high degree of difficulty.

Romney did not announce or emphasize unexpected policy — which is generally not the purpose of debates. But his summaries of existing approaches were crisp and comprehensive. He not only pronounced, he explained.

And he employed vignettes like political chess moves — a little hopelessness in Missouri, a little despair in Wisconsin, a little disillusionment in New Hampshire. Romney constantly and seamlessly humanized his arguments. He even outlined a philosophy of government that includes compassion for the needy — probably a fragment of his prepared response to the 47 percent challenge that never came.

If women voters in battleground states were watching, they didn’t see the stereotype they expected.

Romney prepared for the debate intensely, and it showed — which means it didn’t show. He had not only practiced his material but internalized and mastered it, leaving a composite impression of ease and authenticity. He seemed eager to make the points he was primed to make — pleased to be finally answering months of accumulated attack ads.

Obama had not debated in years, and it also showed. He is a political orchid, thriving best in a hot, wet atmosphere of praise.

Presented with serious, sustained criticism, he first seemed puzzled that his idiom wasn’t working properly. Then came the avalanche of tweeted adjectives: annoyed, grim, unhappy, disengaged, glaring, defensive.

For me, the low point came when he protested, “I’m going to make an important point here, Jim.”

Show, as they say, don’t tell. Obama’s words were instantly forgettable. But his performance will be remembered, studied and mocked for its body language. He looked down. He looked away. It was the surrender of the averted gaze.

The best news for Romney is this: He rose to the most difficult moment. When the need was greatest, and the stage was largest, he was exceptional. It is one of the things that presidents do.