Sen. Bernie Sanders (I., Vt.) has an interesting analysis of why President Obama has had trouble moving his agenda.

Sanders says an inspired grassroots coalition swept Obama to victory. But after his election, the president devoted himself to the work of being a president without tending enough to the grassroots activists who had helped elect him.

"Obama got elected and said: 'Hey, thank you so much for your help. I'll take it from here.' And I think that was a tactical mistake," Sanders noted when he spoke to the editorial boards of the Inquirer and Daily News in April. "I think he should have mobilized the American people in a way that he did not do."

You have to wonder if those activists could have helped the president come up with a better version of the Affordable Care Act, or helped him have a smoother ride implementing the Wall Street Reform and Consumer Protection Act, or pushed him to fight global warming, raise the minimum wage, and lower the role of money in politics earlier and harder during his tenure.

That's where Sanders' analysis gets more interesting because it offers a clue to his next steps. In his speech Monday at the Democratic National Convention, he told his supporters that they accomplished much in this campaign, including moving Democrats on college affordability and raising the minimum wage but that the work must continue.

Sanders' candidacy ended without much proof that he could implement his policies, especially working through a Republican-controlled Congress that has blocked movement on issues he supports. But Sanders' ability to inspire, to organize, to give voice to the grinding frustrations of millions who are having trouble just getting through the week, was proven masterfully during his campaign.

His life of activism came through. Born in Brooklyn, he attended Brooklyn College and finished at the University of Chicago, where he participated in sit-ins against segregation and for students' rights in the early 1960s. He's lived on a kibbutz and protested the Vietnam War. After several tries as an independent candidate, he won the mayoralty of Burlington, Vt., in 1981. An activist mayor, he worked for affordable housing, environmental protection, and child care, among other improvements. In Congress, he was a voice in opposition to the Iraq war and tax cuts for the wealthy, and in favor of providing decent health care to veterans.

He can lead like-minded activists but less certain is whether he and his supporters can take the next step together by transforming their organization into a political force that elects like-minded candidates to office.

That clearly is his goal.

"Election days come and go. But the struggle of the people to create a government which represents all of us and not just the 1 percent - a government based on the principles of economic, social, racial, and environmental justice - that struggle continues," he said at the Wells Fargo Center.

Unfortunately, some of his sympathizers aren't ready to understand that change takes time, often generations of starts, setbacks, and stops. Those followers probably will fall away because the work ahead is hard and the victories few. And they will leave unless they learn that activism is a lifelong pursuit that reaches beyond a few months on a political campaign.

But many will stay in this progressive movement. Their story doesn't have to end now that the Democrats have packed up and left Philadelphia. Sanders can take his place in history by tending to the grassroots and keeping them inspired.