But if events of recent days hold true, they will be facing much more than that.

Rallies in support of immigrants around the country have attracted crowds that have astonished even their organizers. More than a half-million demonstrators marched in Los Angeles on Saturday, as many as 300,000 in Chicago on March 10, and — in between — tens of thousands in Denver, Phoenix, Milwaukee and elsewhere.

One of the most powerful institutions behind the wave of public protests has been the Roman Catholic Church, lending organizational muscle to a spreading network of grass-roots coalitions. In recent weeks, the church has unleashed an army of priests and parishioners to push for the legalization of the nation's illegal immigrants, sending thousands of postcards to members of Congress and thousands of parishioners into the streets.

The demonstrations embody a surging constituency demanding that illegal immigrants be given a path to citizenship rather than be punished with prison terms. It is being pressed as never before by immigrants who were long thought too fearful of deportation to risk so public a display.

"It's unbelievable," said Partha Banerjee, director of the New Jersey Immigration Policy Network, who was in Washington yesterday to help plan more nationwide protests on April 10. "People are joining in so spontaneously, it's almost like the immigrants have risen. I would call it a civil rights movement reborn in this country."

What has galvanized demonstrators, especially Mexicans and other Latin Americans who predominate among illegal immigrants, is proposed legislation — already passed by the House of Representatives — that would make it a felony to be in the United States without proper papers, and a federal crime to aid illegal immigrants.

But the proposed measure also shows the clout of another growing force that elected officials have to reckon with: a groundswell of anger against illegal immigration that is especially potent in border states and swing-voting suburbs where the numbers and social costs of illegal immigrants are most acutely felt.

"It's an entirely predictable example of the law of unintended consequences," said Joshua Hoyt, executive director of the Illinois Coalition for Immigrant and Refugee Rights, who helped organize the Chicago rally and who said he was shocked by the size of the turnout. "The Republican party made a decision to use illegal immigration as the wedge issue of 2006, and the Mexican community was profoundly offended."

Until the wave of immigration rallies, the campaign by groups demanding stringent enforcement legislation seemed to have the upper hand in Washington. The Judiciary Committee was deluged by faxes and e-mail messages from organizations like NumbersUSA, which calls for a reduction in immigration, and claims 237,000 activists nationwide, and the Federation for American Immigration Reform, which has long opposed any form of amnesty, including a guest-worker program advocated by President Bush.

Dan Stein, president of the federation, acknowledged the unexpected outpouring of protesters, but tried to play down its political significance. "These are a lot of people who don't vote, can't vote and certainly aren't voting Republican if they do vote," he said.

But others, noting that foreign-born Latinos voted for President Bush in 2004 at a 40 percent greater rate than Latinos born in the United States, said that by pursuing the proposed legislation, Republican leaders might have squandered the party's inroads with an emerging bloc of voters and pushed them into the Democratic camp.

The Pew Hispanic Center estimates that of more than 11 million illegal immigrants, 78 percent are from Mexico or other Latin American countries. Many have children and other relatives who are United States citizens. Under the House measure, family members of illegal immigrants — as well as clergy members, social workers and lawyers — would risk up to five years in prison if they helped an illegal immigrant remain in the United States.

"Imagine turning more than 11 million people into criminals, and anyone who helps them," said Angela Sanbrano, executive director of the Central American Resource Center of Los Angeles, one of the organizers of Saturday's rally there. "It's outrageous. We needed to send a strong and clear message to Congress and to President Bush that the immigrant community will not allow the criminalization of our people — and it needed to be very strong because of the anti-immigrant environment that we are experiencing in Congress."

Like many advocates for immigrants, Ms. Sanbrano said the protesters would prefer that Congress passed no immigration legislation rather than criminalizing those who are here without documents or creating a guest-worker program that would require millions to go home.

In a telephone briefing sponsored last week by the National Immigration Forum, the Rev. Samuel Rodriguez Jr., president of the National Hispanic Association of Evangelicals, warned that elected officials would pay a price for being on the wrong side of the legislative battle.

"We are talking to the politicians telling them that the Hispanic community will not forget," he said. "I know there are pure hearts that want to protect our border and protect our country, but at the same time the Hispanic community cannot deny the fact that many have taken advantage of an important and legitimate issue in order to manifest their racist and discriminatory spirit against the Hispanic community."

Seventy of the nation's 197 Catholic dioceses have formally committed to the immigration campaign since the United States Catholic Conference of Bishops began the effort last year, and church officials are recruiting the rest.

Meanwhile, priests and deacons have been working side by side with immigrant communities and local immigrant activist groups.

Leo Anchondo, who directs the immigrant campaign for the bishops' conference, said that he was not surprised by the size of the protests because immigration advocacy groups had been working hard to build a powerful campaign. "We hadn't seen efforts to organize these communities before," Mr. Anchondo said. "It's certainly a testament to the fact that people are very scared of what seems to be driving this anti-immigrant legislation, to the point that they are coming out to make sure they speak and are heard."

Last night in downtown Los Angeles, Fabricio Fierros, 18, the American-born son of mushroom-pickers who came to the United States illegally from Mexico, joined about 5,000 Mexican farmworkers gathered for a Mass celebrating the birthday of Cesar Chavez.

"It's not fair to workers here to just kick them out without giving them a legal way to be here," Mr. Fierros said, "To be treated as criminals after all the work they did isn't fair."