Forgive me for comparing Barack Obama to Daffy Duck, but let me spin out an analogy.

There’s a Looney Tunes cartoon in which Daffy is desperate for applause from an audience in a theater. Everything Daffy does is met with silence until he swallows dynamite and gasoline and blows himself up. The crowd goes wild. As Daffy’s ghost ascends to the heavens, Bugs Bunny tells him the audience is screaming for more.

Says Daffy: “I can only do it once.”

It’s beginning to occur to liberals that Barack Obama also could only do it once — and that, like the unfortunate Daffy, the way he won the presidency the first time may have made it impossible for him to do it a second time.

He garbed the presidency in the clothing of hope and change, but after Mitt Romney stripped him bare in last Wednesday’s debate, he stands as naked before his own supporters as he has been to those who never bought into him.

All non-incumbent campaigns promise hope and change, but Obama took the promise to a new level of absurdity. He suggested that a vote for him would literally transform the Earth.

“Generations from now,” he said on the night he put Hillary Clinton away during the Democratic primary season in 2008, “we will be able to tell our children that this was the moment. . . that the rise of the oceans began to slow and our planet began to heal.”

And there was some sense on the part of Obama and his speechwriters that he was committing an almost comic act of hubris in making this claim, since he began the section in which he spoke it with the words, “I accept this challenge with humility and knowledge of my limitations.”

If that had been a sworn statement, Obama would be in jail for perjury now, because he is a stranger to humility and does not believe he has limitations.

He demonstrated that in his first two years in office, when he acted and spoke as though his victory meant he could and should do anything he wished.

The problem for him is that he has acted the same way in his third and fourth year in office — when he has had every reason to conduct himself with the humility and sense of limitations he falsely claimed his victory had instilled in him.

If he loses in November, this will be the reason why. He believed in his own hype about hope and didn’t change when change could have transformed his presidency into an undoubted two-term proposition.

There’s an argument abroad since the debate that Obama was so awful he revealed the truth about himself — a truth papered over by the extraordinary circumstances in 2008 that helped him win the presidency. He is not nearly as eloquent, masterly, and smart as he and his sycophants think he is, and now everybody is onto him. So the argument goes.

That may be true in part, but only because nobody could have been as eloquent, masterly and smart as Obama and his toadies have believed in their most ludicrous moments.

The implicit premise of his 2008 victory was that if you voted for Barack Obama you were not just making a choice between two candidates but making history, changing history, affirming something high and grand and noble. A vote for him elevated you.

As a strategy, it was brilliant. It turned a mere vote into a transcendent social and cultural statement. But no human being could live up to such preposterous expectations, and no human being should ever believe anything so absurd about himself. Obama did and does.

For a while, that astounding self-esteem propelled him to do extraordinary things. Except for FDR, no one changed America and the relation of the government to the economy faster than Barack Obama did.

Obama spent colossal sums of government money, exploding the national debt in the midst of a recession. And he insisted on involving of government in the national economy in unprecedented ways.

The 2009 stimulus cost $867 billion. The auto bailout that same year that led to partial government ownership of two of the largest companies in America cost a minimum of $25 billion.

The price tag for ObamaCare, passed in 2010, is at least $1 trillion and probably close to double that — and it effectively will put the national health-care system under the control of the federal government.

Ninety-million people went to the polls in November 2010 and let him know what they thought of all that. The results were astonishing — a 63-seat shift to the Republicans in the House of Representatives.

He called it a “shellacking.” He could have heeded its meaning and moderated his approach. That was what Bill Clinton did after his drubbing in 1994.

But not Obama. His astonishing self-esteem was unaffected. He continued to run as a big-government liberal, eager to raise taxes on the rich in pursuit of fairness, eager to involve government still further in the economy.

The Obama who showed up at the debate on Wednesday night before an audience of 70 million was a candidate unprepared for the fact that he might have an opponent who could challenge his governance effectively.

Even now he seems to think that making an actual case for a second term is beneath him. Instead, he’s just taken to saying Romney is a liar and that he likes Big Bird.

His answer at the Democratic convention in September was to ask for . . . more hope. “Yes, our path is harder,” he said at the Democratic convention, “but it leads to a better place.”

He’s going to have to do much, much better than that in the coming weeks or, like Daffy Duck, his career is going to go up in smoke.

jpodhoretz@gmail.com