"We can end a war. ... We can save the planet. ... We can change the world."

A few years ago, if you had suggested that a leading contender for the Democratic presidential nomination consider airing these sentiments in ads broadcast during the Super Bowl, most political pundits would have said you were insane. The Super Bowl, watched by nearly a third of the U.S. population, is about football, beer and machismo. It's not about the anti-war movement, the environmental movement, the anti-poverty movement or peace, love and understanding.

But Sunday, Barack Obama aired a 30-second Super Bowl ad in some television markets that drew unabashedly on the iconography of the American left. The ad offered images of rallies and protest marches, of poverty and environmental destruction, of the devastation of war and of beaming, hopeful, multiracial crowds. Broadcast not to a niche demographic of activist students or South Carolina blacks but to a cross-section of football fans, the message was nostalgic and idealistic.

The Obama ad highlights a recent sea change in Democratic politics, one that's impossible to understate. A few short years ago, Democrats were on the defensive. On national security issues, the party's Beltway power brokers anxiously debated how best to look "tough." That led easily into a depressing sort of "me tooism," as Democrats competed to show that they weren't the wimpy, soft creatures of Republic caricature but hard, chest-beating types, willing to embrace wars, abandon civil liberties and kill terrorists deader than dead.

On domestic issues, Democrats were also running scared. Most congressional Democrats voted to support Bush's ruinous 2004 tax cut, for instance. And in general, Democrats did their darnedest to avoid using words or images that would remind the average American of the 1960s. The conventional wisdom was that bringing up the anti-war movement or the women's movement or race or poverty would be a gift to the right.

No more. All of a sudden, Democrats are on the offensive. "Change" isn't just this year's most ubiquitous campaign slogan, it seems to be something that's already happening out there in the real world, in small towns, on college campuses and, yes, even at Super Bowl parties.

Who knows what caused the shift in mood? Iraq? Katrina? Global warming? Rising income inequality? Disgust with Bush and Cheney? Whatever the causes, Americans seem eager to reclaim a spirit of idealism that many thought ended with the 1960s, to embrace a heritage that acknowledges conflict and struggle but also hope and progress.

Obama's Super Bowl ad represented a gamble: a bet that the symbolism of past social movements is now more likely to give Americans a thrill than a chill. And the matter-of-factness with which his ad was greeted — and Obama's electoral success so far — suggest that his campaign correctly read the national mood.

Hillary Rodham Clinton's campaign also has recognized that Democrats are operating in a changed landscape. Clinton increasingly seeks to appeal to the same idealistic spirit as Obama, the same nostalgia for those moments when changing the world seemed truly to be within our grasp, the same conviction that we again have an opportunity to reshape our world.

Today, the arguments between the two candidates are over who is best placed to bring about the seismic change that both candidates assume voters want. Is it Obama with his multiracial background, his youth, his broad appeal and his lack of baggage? Or is it Clinton, a woman who can claim to have learned some painful lessons about when to compromise and when to stick to her guns?

It's far too soon to say if the newfound spirit of idealism that's sending voters (including many independents) to the Democratic primaries in record numbers will endure, paving the way for an era of energized new social movements and reforms. But I would bet that we really have turned a page. On the Republican side, too, there's a palpable desire for a candidate who doesn't fit into a rigid ideological box, one who can tap into and reflect our best instincts instead of our most craven fears.

Whether the idealistic yearning for change endures probably has little to do with who wins the Democratic nomination (or even the White House). Losses can galvanize social movements as much as victories, and whoever wins the White House will be president of an America different from the one that greeted Bush's inaugurations in 2001 and 2005. It will be a more hopeful, less partisan nation, one united in its rueful awareness of the ways the Bush presidency went wrong, a nation more ready to get to work to put things right.

Rosa Brooks is a professor at the Georgetown University Law Center. She wrote this commentary for the Los Angeles Times.