By Gregory Rodriguez: All last week, commentators recounted the dramatic swing in the national mood between this January and last.

All last week, commentators recounted the dramatic swing in the national mood between this January and last. In 2009, President Obama was talking about bending the arc of history. In 2010, it doesn't look like he can bend enough arms to get health-care reform through Congress.

Americans are not happy. We're on the downside of a familiar cycle: the bitter disillusionment that follows outsized hope. What could be accomplished in Washington isn't matching our expectations. And although Obama's fall has been steep and hard, he's not alone. Traditionally, a new president's party loses ground in the election that follows his inauguration. What's surprising is that we never seem to tire of the vaunted hope that gets the cycle going in the first place.

What's going on? Are we schizophrenic? Gluttons for punishment? Are we so addicted to the possibilities that, even when reality mugs us, we can't wait to dream again? Why hasn't the mountain of disappointing political realities — presidents, parties, causes and processes — turned us into a nation that stops caring?

The answer to these questions lies in the contradictory heart of the American character. On the one hand, we pride ourselves on our idealism. On the other, we fancy ourselves sturdy realists. If the U.S. were a cartoon character, it'd be a cheerful fellow with his head in the clouds and his feet planted squarely on the ground.

This contradiction between idealism and realism is not just an incidental quirk in our collective personality. It's one of the primary tensions that drive us. Although the manic back-and-forth is frustrating, even alarming, one could argue that it's also the source of our dynamism as a nation. Harvard historian Bernard Bailyn has identified the creation of this fundamental tension as one of the great legacies of the founders. He hypothesizes that if Thomas Jefferson's intellectual idealism had not been tempered by the "hardheaded pragmatism" he learned as a "man of business," he wouldn't have played such a powerful role in history.

"The blending of realism and idealism," Bailyn writes, "permeates the entire history of the Revolutionary era." The Constitution, with its checks and balances, is as much about controlling liberty, and the will of the people, as it is about exercising it, and as such it "reflects precisely the creative tension between idealism and realism in American public life."

Bailyn's characterization is fine as far as it goes, but it doesn't quite capture how annoying the flip-flopping between the two poles can be. Novelist and poet Robert Penn Warren described realism as "the burr under the metaphysical saddle" of American idealism. "That saddle's going to jump now and then," he wrote, "and it pricks."

In other words, it's uncomfortable to be so rudely awakened from our lofty dreams. We express this discomfort in our art, literature and humor. Americans love to debunk, satirize and ridicule. When Stephen Colbert pokes fun at the devilishness of a sanctimonious politician, he's just trying to close the gap between the spin and the facts.

Another way we close the gap is through incessant reform. The late great political scientist Samuel P. Huntington argued that the tension between the ideal and the real stoked the American "moral motive for reform." Activists on all sides routinely seek to draw stark contrasts between our ambitions for good and our painful imperfections. The ability to expose hypocrisy, or even just the inevitable shortfall when our reach exceeds our grasp, can determine the success or failure of any given cause. When the civil rights movement forced us to compare discrimination to our ideal of equality, something had to give.

Huntington called the tension between promise and reality our nation's "distinguishing cleavage." If it were to disappear, he wrote, the U.S., "as we have known it, will no longer exist." And to those who would point to the gap between what we strive for and what we achieve to prove that the American promise is bogus, Huntington responded: "America is not a lie: It is a disappointment. But it can be a disappointment only because it is also a hope."

I don't disagree with his logic, but the cycle between the two these days seems to be in overdrive. It's hard to believe it won't eventually wear us down. I don't know if it has the potential to turn us into a nation of cynics, but it seems to me that it could cause us to lower our sights and goals.

Was the Obama hype way over the top? No question. And any reasonable person could have predicted his fall from grace. Nonetheless, there is something irresistible about a country so willing to believe, despite its disappointments, that it tried to walk with its head in the clouds.

Gregory Rodriguez is a columnist for the Los Angeles Times. Readers may send him e-mail at grodriguez@latimescolumnists.com.