Editor’s Note: Each week we cross-post an excerpt from Katrina vanden Heuvel’s column at the WashingtonPost.com. Read the full text of Katrina’s column here.

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.

Editor’s Note: Read the full text of Katrina’s column here.