WASHINGTON — In a conference room at the Democratic headquarters, President Obama has been preparing for the debate next week, but the reviews of his staff are already in. Too long, they tell him. Cut that answer. Give crisper explanations. No one wants a professor; they want a president.

Hundreds of miles away in New England, Mitt Romney’s team has been working to make sure he avoids coming off as a scold. His sparring partner, Senator Rob Portman of Ohio, channeling Mr. Obama, has gone after him repeatedly, to the point of being nasty. The goal is to get Mr. Romney agitated and then teach him how to keep his composure, look presidential.

With more than 50 million people watching and the presidency at stake, the candidates will meet for their first debate on Wednesday at the University of Denver, and both are cramming like college students before an exam. But it is not enough to pore through the voluminous briefing books. Victory may come down to a single exchange, or a single impression, an answer that comes off as too edgy or, conversely, as too long-winded.

Mr. Obama’s team records his practices to sharpen his responses so that they connect on a more visceral level with the television audience. One of Mr. Romney’s aides calculated his words-per-minute rate in the primary campaign debates to break him of the habit of feeling that he needs to rattle off the most statistics.