Can Obama stop a Romney victory?

Editor's note: Alex Castellanos, a CNN contributor, is a Republican consultant and the co-founder of Purple Strategies. Follow him on Twitter: @alexcast

(CNN) -- In a presidential campaign laden with historic twists, the biggest shock may be yet to come: With three weeks to go, the first of two remaining debates tonight, and most Americans yet to cast their ballots, the final surprise may be that there are no surprises left.

This campaign for president may be over. There is a good chance that the Denver debate was the watershed that carries this election to its conclusion. If so, Barack Obama has only himself to blame. Yes, the polls show a very close race, but the momentum has belonged to Romney since his superior showing in the first debate.

Alex Castellanos

Long ago, Team Obama displayed a heartbreaking lack of faith in its candidate and his prospects. Abandon hope, all ye who campaign here; they never tried to get Barack Obama re-elected president. Constrained by the paucity of their achievements and the poverty of their ideas, perhaps they felt they had no choice: We can't win, they concluded, but maybe we can stop the other guy from beating us. Obama's thin campaign only tried to stop his opponent from winning.

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Until this last month, the president's "This is a one-man race" marketing philosophy was working -- but only with the assistance of an opponent who ran as if his shoes were tied together. Mitt Romney had allowed Obama to paint him as an unacceptable alternative. He also helped the president with 47% of the brushstrokes.

Team Romney's inability to introduce their candidate as a good and decent man has frustrated Romney's core supporters, who've longed to hear more about their man's munificence and compassion. Romney's campaign seemed equally unable to acquaint voters with the transformational business leader who, as Ann Romney describes, has routinely done things others thought "impossible." All that changed two weeks ago in the pure Denver air.

What millions of dollars of ads had not been able to do, Mitt Romney did for himself. The Romney who showed up in the first debate did not, as Obama had led voters to expect, have horns and tail. Instead, he was the most unexpected candidate of all: a reasonable alternative to good-hearted but flailing president. In debate one, Romney even admitted he was having fun, an emotion we were not led to think he was programmed to express.

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In the presence of an acceptable Republican replacement, the only theory of the president's campaign, that voters had no choice but Obama, began to break down. Voters began to suspect there was no alternative, not to Obama, but to Romney. Suddenly, Barack Obama was the man who wasn't there.

If the 2008 election was about hope and change, the 2012 contest has always about strength and certainty. In these turbulent and uncertain times, the most unacceptable quality in a leader is weakness.

In one flashing moment in a primary debate, Tim Pawlenty refused to say to Romney's face what he had said absent his presence, that Romney had created "Obomneycare." Without hesitation, voters neutered Pawlenty.

General election voters may have seen similar weakness in Obama, as he sleepwalked with unmasked indifference through his greatest political challenge. If our president wasn't equal to the defining moment of his own re-election, how can we trust him to rise to the test of fixing the economy?

Now, with only three weeks to go, Obama's campaign marinades in its powerlessness. Doubts metastasize as the administration moans that no one asked the president to protect our Libyan ambassador, as if it had not been his responsibility. He's started losing female voters, treating women as no more than a constituency of sexual organs and then hiding behind his secretary of state's skirt. He pretends to protect women, Hispanics, seniors, when he cannot protect himself.

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Debates are the most primitive moments in our democracy. In these primal contests, we test those who would lead our country. We send our gladiators into the coliseum to see if they have the authority to command an unexpected moment, satisfying our blood thirst for strong leadership. There is no bigger test.

Tonight we should expect a solid performance from both men. Romney has been doing "Ask Mitt Anything" Town Halls for five years. On the stump these days, he connects with ease, telling stories of the people he's met, moved by their hopes and suffering. His growth is evidence that our campaign process is not only built to pick presidents but to shape them.

Obama is gifted in the town-hall debate format, too. His performance against John McCain was stately; but one debate alone is probably not enough to rebuild what he has destroyed, the image of constancy and strength.

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It takes only a moment to reveal feebleness. It requires many to reestablish certainty. One good debate tonight will help re-energize Obama's base but not our country's confidence in him.

Whether victory belongs to Romney or Obama, we do not know but this cake may be baked: The Denver debate may have eradicated the opportunity for both candidates to change the outcome of this election.

Mitt Romney may have already won this election: Even if the president has a strong performance tonight, he failed us on the economy, failed us in Libya, failed us in Denver.

How can we trust that Barack Obama won't fail us again?

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The opinions expressed in this commentary are solely those of Alex Castellanos.