The immigrant community, particularly Hispanics, has rarely if ever been so mobilized in such a monolithic bloc - as Monday's marches, rallies, and boycotts made clear.
So where does the movement go from here?
Sign after sign carried through Chicago streets on Monday during a massive immigrant-rights rally said: "Today we march, tomorrow we vote." Whether the slogan comes true will depend on how well the coalitions behind the widespread May 1 events can transform street power into political clout.
For now, most immigrant organizers are basking in the turnout for the marches: 400,000 people both in Chicago and Los Angeles; between 20,000 and 40,000 each in Houston, San Francisco, New York, and Orlando, Fla. Smaller marches took place in other cities across the country.
But taking the next step may prove to be much harder than organizing protests against reform legislation many immigrants see as overly harsh.
For one thing, the marches may be polarizing American opinion on immigration reform - even galvanizing those who want more restrictive laws. For another, numbers of protesters don't equal numbers of voters: Immigrants, legal or illegal, can't vote. Only immigrants who are naturalized US citizens have that right, and among eligible Hispanic voters, turnout rates have traditionally been low.
"A lot depends on how successful they are at registering people and getting them to vote," says Gabriel Escobar, associate director of the Pew Hispanic Center. "There are many, many issues on which Hispanics are divided, but the one thing that has the potential of bringing them all together is the perception that there is this threat against them. That's the thing that historically creates movements."
Much will depend on grass-roots groups that have been the mobilizing force thus far behind the demonstrations, say Mr. Escobar and others. Chiefly, can they continue to reach people, effectively organize them, and turn their frustration about a bill the House of Representatives approved in December - which makes illegal status in the US a felony rather than a civil offense, and makes it against the law to help undocumented immigrants already here - into political action?
"Since the very first organizing steps in early March, a number of groups have started to look at how to link that advocacy on the street to advocacy in Congress," says Clarissa Martinez, director of state policy for the National Council of La Raza. More important, she says, is to "make sure that that level of community organizing and response is also linked to electoral organizing."
Ms. Martinez is encouraged by the number of grass-roots and community groups that are already concentrating on voter registration and naturalization. She expects that the mobilization will eventually lead to a spike in naturalization - as happened after California's Proposition 187 in 1994.
She's also heartened to see the large numbers of young people who have turned out at rallies, many of whom are American citizens and make up the next generation of Hispanic voters. Normally, "a lot of people just feel detached from elected officials and political leadership and assume that politics don't matter in their daily lives," Martinez says. But on this issue, "the relevance is in your face."
Despite organizers' enthusiasm, immigrant groups face several hurdles in their attempt to sway politicians and public opinion. Chief among them is a historic pattern of Hispanic nonvoting. In 2004, 47 percent of eligible Latino voters in the US went to the polls, compared with 67 percent for whites and 60 percent for blacks, according to research from the Pew Hispanic Center. They accounted for 6 percent of all votes cast - up slightly from 5.5 percent in 2000.
Moreover, division is emerging among activists about the best strategies to use. In Los Angeles, organizers created a "Day Without Immigrants" - a work boycott that closed many businesses and interrupted many services - while other parts of the country elected to stick to marches or vigils. Some say a backlash to the demonstrations is inevitable.
In an NBC News/Wall Street Journal poll at the end of April, 57 percent of respondents said it would hurt immigrants' cause if thousands of them did not show up to work on May 1 in protest of immigration policy, compared with 17 percent who thought it would help it.
"People are now less favorable toward amnesty than they were before," says Ira Mehlman of the Federation for American Immigration Reform, which opposes illegal immigration. "It is the perception that we are moving toward mob rule. You can't force the American people to do something. It is a form of extortion, and people don't like to be extorted."
Immigrant-rights advocates, however, say the peaceful, family-oriented nature of the marches has helped to tamp down any backlash. "Boycott aside, [activists and protesters have] learned to minimize disruption," says Michael Fix of the Migration Policy Institute. "They've marched at times that are not going to disrupt the economy. They're doing it to increase political acceptance - they don't want to make those tensions higher."
The mobilization, he says, should be the first step in a process that moves people to become naturalized citizens and, ultimately, earns the attention of politicians and the public.
Monday's marchers, though, weren't asking such nuanced questions. They were simply reveling in the display of immigrant support, certain that it must influence federal lawmakers. Chicago's Grant Park was a sea of people, many with their families, wrapped in US flags or Mexican flags, waving signs that read, "The pilgrims didn't have papers" or "Muslims for the American dream."
"It's been so inspiring to see the marches - everyone coming out to voice their opinion, to voice their right to speech," said Elia Baez, a loan officer who carried a US flag and a sign that read, "Justice for Immigrants." Ms. Baez had also attended the March 10 rally, and came to this one with her mother, sister, husband, and brother.
A few organizers are already talking about ways the turnout can be translated into support for other issues: education, housing, adult English classes, or labor rights.
"The Latino community was sleeping," says Claudia Lucero, an organizer with Durango Unido, one of the many associations of Mexican hometowns that serve as organizing forces in Chicago. "I don't think we've seen anything like this in US history."
• Chris Gaylord helped report this story.