In the weeks leading up to the March 21 demonstration for comprehensive immigration reform, organizers were careful to tamp down turnout expectations, stating only that "tens of thousands" would descend on Washington. It was just a few days before the event–with reports of countless buses heading toward the nation’s capital–that they hinted that the crowd could reach 100,000.

So, on a beautiful spring day, with an estimated 200,000 people breaking into raucous chants of "¡Sí Se Puede!" on the National Mall, there was reason for optimism. "I walked and walked and walked," a man behind me excitedly told his wife and daughter in Spanish. "Finally I found the end of the crowd–way back there!" He pointed toward the Washington Monument and the general direction of his travels, but all his family could see was a solid wall of people, many of them teenagers proudly wearing Undocumented and Unafraid T-shirts.

Yet while the March for America was, as Deepak Bhargava of the Center for Community Change told the crowd, "the biggest mobilization on any issue since Barack Obama was inaugurated president," it was also something of a disappointment. "My wife said it didn’t even make the news," a friend told me as we traveled back to New York City that evening. "How is that possible?" It was possible, of course, because television cameras were glued to the negotiations in the House over the healthcare bill, certainly a momentous occasion. As a result, many Americans had no idea that nearly a quarter-million people–overwhelmingly Latino immigrants–had turned out to press for action.

But for those attending the demonstration, it was clear that the same energy that animated marchers in 2006 is definitely back. The feeling among the crowd was a healthy mix of hope and anger, along with an unmistakable sense of mission.

"Obama promised us he would be on our side," said Juan Carlos, a slender Mexican immigrant who had come from Chicago with members of his church, referring to the president’s pledge to move on immigration reform in 2010. "We’re here to make sure he keeps his promise." I heard the same comment from a number of people, and countless homemade signs echoed the sentiment–but this wasn’t message discipline as much as a widely and deeply held belief. Now, following the large but largely overlooked demonstration, the trick will be harnessing that belief into sustained political action in the coming months.

Despite the unfortunate timing, it’s important to remember how quickly grassroots organizing and sharp public criticism have forced Obama to shift his tone. Less than two months ago, in his State of the Union address, Obama made only the most general allusion to immigration reform, stating blandly that we should "secure our borders, enforce our laws and ensure that everyone who plays by the rules can contribute to our economy and enrich our nation." Translated, it was easy enough to read this as: immigrants’ rights are not a priority.

On Sunday afternoon Obama’s tone was decidedly more urgent when he addressed the crowd via a prerecorded video appearance on the large screens. "I pledge to do everything in my power to forge a bipartisan consensus this year," he said. "This won’t be easy, and it won’t happen overnight."

That’s a lesson immigrants have already learned, of course, beginning with Obama’s initial reluctance to champion reform and the increase in deportation proceedings during his administration. And it’s clear that the march was only an opening salvo. Already, plans are in the works for large demonstrations nationwide on April 10 and again on May 1; the second wave of actions will coincide with the release of a report card by the Reform Immigration for America Campaign grading politicians on where they stand on the issue–an evaluation that should command attention, given the growing clout of Latino voters.

Remaining vigilant is also a task for advocates who are dissatisfied with the proposed reform framework and oppose the inclusion of an expanded guest-worker program prone to labor abuses and the introduction of nefarious-sounding measures like the "biometric Social Security cards" recently praised by Senators Lindsey Graham and Charles Schumer in the Washington Post. And beyond specific policy improvements, progressives need to push back against language in the debate that tends to paint undocumented immigrants as guilty of anything but attempting to improve their lot. As Graham and Schumer wrote in the Post about their proposed legislation, undocumented immigrants "would be required to admit they broke the law and to pay their debt to society." There’s another case to be made: we owe a debt to them.

But however much one might feel the current proposals fall short, it can’t be denied that the massive turnout breathed new life into a movement that seemed to have fallen by the wayside. Another heartening sign was the diverse range of groups represented–from organized labor, churches and community groups to the NAACP and the National Urban League. The involvement of the last two organizations is especially important, highlighting the fact that immigrants and African-Americans are both served by legislation that brings millions of workers out of the shadows, which will raise workplace standards for all.

As organizers are clearly aware, with unemployment officially at nearly 10 percent, it is crucial to tackle head-on the notion that the economic downturn should take immigration reform off the table. Indeed, plenty of recent analysis points to immigration reform playing a positive role in jump-starting an ailing economy. The libertarian Cato Institute issued a report last year concluding that reform would add roughly $180 billion to the economy per year. In January a study by UCLA’s Raúl Hinojosa-Ojeda, released by the Immigration Policy Center and the Center for American Progress, concluded that reform would add $1.5 trillion over the next decade while boosting wages for native-born and newly legalized workers.

It was impossible not to contrast Sunday’s march with the ugliness of the previous day, when a thousand tea partyers angrily denounced Obama at a "Kill the Bill" rally against healthcare reform, at the Capitol. When African-American Representative Emanuel Cleaver walked by, one of the protesters spit on him; others shouted "nigger" at Congressman John Lewis, a civil rights icon who said the episode reminded him of another time. That same day, more enraged protesters screamed "faggot" at Barney Frank. Twenty-four hours later, a crowd 200 times the size and infinitely more diverse cheered the words of Representative Luis Gutierrez, who introduced reform legislation in December and has been a leading advocate for national legislation. "We’ve listened quietly," he told them. "We’ve asked politely. We’ve turned the other cheek so many times our heads are spinning. It’s time to let immigrants come out of the shadows into the light and for America to embrace them and protect them."