Mr. President, why so angry? What’s up with that gloomy edge and the sour mood?

Aside from all the bewildering back-and-forth about Libya, Iran, apology tours, and auto bailouts, by far the most important aspect of the third and final presidential debate involved its atmospherics—which candidate came across as more calming and confident, more plausible and reassuring as commander-in-chief? Which of the contenders, in other words, emerged from their final confrontation looking like a leader, frontrunner, and ultimate winner?

In that regard Barack Obama helped himself with his fluent command of foreign affairs and passionate engagement with the disparate subjects under discussion. But he hurt himself with a chip-on-the-shoulder demeanor and attack-dog aggressiveness against Mitt Romney. On several occasions the GOP nominee reasonably responded that “attacking me is not an agenda.”

Debate viewers watched an odd spectacle in which the president of the United States, the most powerful man on earth, devoted a considerable portion of his prime opportunity to discuss his nation’s place in the world, to smearing reviews of random speeches, interviews, and position papers by his unelected opponent.

As Jimmy Carter discovered in his single disastrous debate with Ronald Reagan, when the incumbent president concentrates his energy on savaging his opponent, he concedes to that rival the status of frontrunner. If he focuses on trying to scare the American people about the prospect of the other guy as president, he implicitly acknowledges that this prospect is both conceivable and imminent. Why else would he want to provoke worry of the opponent’s proposals for the next four years rather than inspiring enthusiasm for his own?

Romney won the debate by offering an image of even-tempered geniality, good-natured self-assurance, and unshakable commitment to cautious, reasonable, and peace-loving leadership. When he repeatedly acknowledged his agreement with aspects of the president’s policies, it reflected strength more than weakness and contrasted powerfully with Obama’s annoying inability to find any aspect of his opponent’s positions or career worthy of praise or even grudging acceptance. In an argument the combatant who takes the more critical and indignant tone almost always ends up as the loser.

This pattern will prove especially potent for the few remaining undecided voters or wavering independents who express their heartfelt yearning for more cooperation and conciliation from our elected officials in gridlocked Washington. Romney addressed that yearning explicitly with his reminder of his own record as a commonsense problem-solver who worked successfully with Democrats as governor of Massachusetts.

In selecting a chief executive to safeguard the nation’s well-being in turbulent times, voters instinctively seek steadiness, cool, and consistency. This last quality gave Romney another edge in this climactic confrontation: the American people saw precisely the same unflappable guy who turned up at debate No. 1 and debate No. 2. In Obama’s case the contrasts and metamorphoses seemed striking—from sleepy Obama to snippy-and-interrupting Obama in the second debate to angry-and-aggrieved Obama in debate No. 3.

These shifts in personal presentation may stem from coming adjustments in strategy and won’t keep the president or his partisans from claiming victory in the final faceoff. After all, he delivered a vigorous, intermittently effective defense of a record in foreign affairs that has, as he claimed, kept the nation safe from major harm.

But in the process he wasted too much time trying to paint a formidable, sophisticated, and unmenacing opponent as a dangerous, warmongering amateur. But the frightening Halloween costume stubbornly refused to fit.

Meanwhile, Mitt may not be ready to don the cloak of presidential command, but in the third debate viewers at least got the chance to check him out as he credibly tried it on.