“It was a David-and-Goliath struggle,” said Roy H. Beck, the president of Numbers USA, who had been preparing for this moment since 1996, when he wrote a book titled “The Case Against Immigration.”

Supporters of the bill included President Bush, the United States Chamber of Commerce, the high-tech industry, the Roman Catholic Church, many Hispanic organizations, farmers, restaurants, hotels and the construction industry.

“The bill had support from the opinion elite in this country,” Mr. Beck said. “But we built a grass-roots army, consumed with passion for a cause, and used the power of the Internet to go around the elites and defeat a disastrous amnesty bill.”

The measure, which died on June 28, would have offered legal status and a path to citizenship to millions of illegal immigrants and created a new temporary worker program while increasing border security.

“Numbers USA initiated and turbocharged the populist revolt against the immigration reform package,” said Frank Sharry, executive director of the National Immigration Forum, a pro-immigrant advocacy group. “Roy Beck takes people who are upset about illegal immigration for different reasons, including hostility to Hispanic immigrants, and disciplines them so their message is based on policy rather than race-based arguments or xenophobia.”

Representative Brian P. Bilbray, Republican of California and chairman of the Immigration Reform Caucus, said, “We’re involved in weekly discussions with Numbers USA and other immigration-control groups as part of a team effort.”

Numbers USA had fewer than 50,000 members at the end of 2004, but now counts more than 447,000, with an increase of 83 percent since January alone.

Turning to the next phase of the debate, those members will push for enforcement of existing laws and new measures to curb the employment of illegal immigrants.

“Our No. 1 legislative goal is to begin a system of mandatory workplace verification, to confirm that every employee is a United States citizen or an alien authorized to work in this country,” said Rosemary E. Jenks, director of government relations at Numbers USA.

The organization wants to reduce immigration — as Mr. Beck says in the subtitle of his book — for “moral, economic, social and environmental reasons.”

He contends that immigrants and their children are driving population growth, which he says is gobbling up open space, causing urban sprawl and creating more traffic congestion.

Moreover, Mr. Beck asserts that immigrants and temporary workers, by increasing the supply of labor, have depressed wages in industries from meatpacking to information technology. Numbers USA has worked most closely with conservative Republicans, but in recent weeks has built alliances with Democrats who share the concern.

Numbers USA keeps a scorecard showing every vote by every member of Congress on immigration-related issues since 2089. The group assigns a letter grade to each member.

Lawmakers who received an A-plus were all Republicans and included Representatives J. Dennis Hastert of Illinois and Tom Tancredo of Colorado, a presidential candidate. The lowest grades — F-minuses — went to Democrats, including Speaker Nancy Pelosi and Representative Joe Baca of California, chairman of the Congressional Hispanic Caucus.

Numbers USA objects to proposals that increase the number of legal or illegal immigrants. It steers clear of debates over the allocation of visas.

“It does not matter to us whether a visa goes to a high-tech worker, a farm worker or the sibling of a U.S. citizen,” Mr. Beck said.

Numbers USA is one of many organizations fostered by John H. Tanton, an ophthalmologist from Michigan who has also championed efforts to protect the environment, limit population growth and promote English as an official language.

Critics like the Southern Poverty Law Center and Representative Chris Cannon, Republican of Utah, have described Dr. Tanton as a father of the anti-immigration movement. Mark A. Potok, a senior researcher at the law center, called Numbers USA the “kinder, gentler side of that movement.”

Mr. Beck said Numbers USA had been independent of Dr. Tanton since 2002. On the group’s Web site, Mr. Beck cautions against “immigrant bashing” and says, “Even illegal aliens deserve humane treatment as they are detected, detained and deported.”

In the fight over the Senate bill, Numbers USA had daily conference calls with conservative groups like the Heritage Foundation and the Eagle Forum.

For tax purposes, Numbers USA has two arms, an educational foundation and an advocacy group that lobbies Congress. Together, Mr. Beck said, they have a budget of $3 million this year, but will probably raise and spend $4.5 million.

Mr. Beck said that in the past the group received about two-thirds of its money from foundations like the Colcom Foundation of Pittsburgh and the Weeden Foundation in New York. Many of these foundations have an interest in conservation.

Numbers USA has raised the rest of its money from individual contributors over the Internet. The group collects detailed information on its members — their ethnic background, politics, religious affiliations, occupations and concerns — so it can choose the most effective advocates on any particular issue.