When President Obama was elected more than three years ago, many progressives had great expectations for what would follow. Many wanted to believe that one person, in one flying presidential leap, could transform the mess our political system had become. That he, alone, could deliver.

Three years later, progressives have learned the hard way that this isn’t, and never will be, the case. Democratic presidents succeed at advancing progressive causes when independent progressive movements push them to do so. Success at the ballot box is not a victory in and of itself. True victory comes when vibrant, sustainable movements create an energy around ideas that the White House has to chase. Those movements can be built on hope, but they are sustained with engagement of the kind that can outlast any given battle, any given term and any given presidency.

This is an idea I sound in my new book, “The Change I Believe In.” The book is about the journey I and millions of Americans took, from the exhilaration engendered by Obama’s election and the first months of his presidency, through the disappointments and frustrations that have followed. It is, above all, about recognizing how transformational change comes about in a system rigged against it.

As I write in the introduction, “The change I believe in is not one that happens in one or two or even three election cycles, or through a top-down approach — no matter who is president.” The president, himself, seems to agree. “I always believed this was a long-term project,” he recently told Steve Kroft on “60 Minutes.” “... reversing a culture here in Washington, dominated by special interests, it was going to take more than a year. It was going to take more than two years. It was going to take more than one term. Probably … more than one president.”

That change, that reversal of culture, almost always comes from outside Washington and from below, from mobilized people and movements of determined idealism and grounded realism. It grows out of the recognition that we are living in a system hardwired to resist reform, and in a political environment that protects the corporate status quo. After three years, the president’s rhetoric — as reflected in his recent masterful Kansas speech infused with the themes of the Occupy movement — shows that movements do play a role in shaping our politics and that they have been essential in pushing the Democratic Party to some of its finest hours.

Great periods of change — the New Deal and the Great Society, for example — took place only after years of effort and many setbacks. So, too, will the change we seek. That means running people for office, from dog catcher to Congress, who channel the 99 percent movement, so that we have allies in positions of influence. It will require savvy organizing and smart strategies by activists — as we see today in the Occupy encampments around the country. And it will require action by principled political leaders — as we’ve seen recently through the “occupy” amendments, such as the one recently proposed by Florida Rep. Ted Deutch and Vermont Sen. Bernie Sanders, which would overturn Citizens United and prohibit corporations from campaign spending.

But the need for a movement does not — and cannot — let those in power off the hook. I believe President Obama doesn’t have to be a kickboxer, but he still has an obligation to explain clearly to a country in economic pain not only what it is we are fighting to achieve, but what can be done to achieve it. As Ezra Klein wrote last week, President Obama has a “last chapter” problem: He has learned to put the problems we face in clear, compelling terms, but he does not fully address how we can overcome them.

As he rightly said in Osawatomie, Kan., the Republican Party has become a bastion of “ ‘you’re on your own’ economics.” His speech was a bold statement about the magnitude of our challenges, and the crisis of income inequality, which he called the “defining issue of our time.” But his modest list of solutions didn’t measure up to the scope of the challenge he defined.

If, as the president suggested, the change we are fighting for takes more than just one presidency, so be it. We’ve expected that all along. But it’s time for the president to more clearly explain to the American people what he believes in — and the lengths he will go to defend and advance those beliefs. That may require him to redefine what it really means to change Washington, and to be honest and forthright about what change will really require.

President Obama likes to quote Martin Luther King Jr., who said that “the arc of the moral universe is long, but it bends toward justice.” But it doesn’t bend by itself. Faced with the roadblocks of the right, perhaps the pragmatic thing to do and the idealistic thing to do are one and the same: We have to build a movement that will push our politics and our current president — and the next one, and the next one, and the next one. As President Obama said, change is a “long-term project.” That means that 2012 must be about more than just his reelection. It needs to be about what comes next, not just who.