They delivered petitions bearing nearly 3,000 signatures, part of a multi pronged campaign, imploring Lott not to "sell out Mississippi to illegal aliens." The office secretary "could barely receive them because the phones were ringing off the hook" with calls protesting Lott's immigration stance, said Wade, a local talk-radio host.

TV, radio and Internet ads condemning Lott had been running for days before the Tuesday visit. By Friday, 1,000 more people had signed the NumbersUSA petition online.

Conservative anger at the Senate immigration bill is at such a pitch that even Republican lawmakers are feeling the heat. Groups like NumbersUSA have been channeling that grass-roots fury and, in doing so, have leaped in size and are playing a larger role in the immigration debate than ever before.

At NumbersUSA, one of the largest and loudest, membership is up 81% since January and donations are soaring. With the immigration bill possibly set to pass or fail in the Senate this week — a crucial vote could come as early as Tuesday — the nonprofit group plans a fierce campaign against the bill and any senator who supports it.

The group will unveil TV ads against Senate Minority Leader Mitch McConnell (R-Ky.) and Sen. Lindsey Graham (R-N.C.). Another ad will target Senate Majority Leader Harry Reid (D-Nev.).

NumbersUSA President Roy Beck expects to contact the group's activist members — who he says now number 419,000 — with dozens of e-mail alerts, many of which he sends at 2 and 3 a.m.

"We're in a war zone right now, so we're drawing down the reserves," said Beck, who describes his battle against pro-bill groups like the U.S. Chamber of Commerce as "an amazing David-and-Goliath thing."

Among those pushing the new measure is President Bush, who appealed for support in his weekly Saturday radio address by stressing revisions to the bill that would require stepped-up enforcement before other provisions could take effect.

Most polls show that though Americans are concerned about border security, a majority favor finding a way to allow most now in the country illegally to gain legal status.

Throughout the immigration debate, senators who back the bill have decried the influence of talk radio, saying it misrepresents the legislation. These lawmakers vow they won't buckle but admit they're feeling pressure.

"Those really pushing for the bill have not been as effective as those pushing against it," Lott said last week, on the same day Wade and his colleagues delivered their petitions.

NumbersUSA members in Mississippi said they had sent Lott about 10,000 faxes and letters since he began supporting the bill in early June. Lott said his phone lines were jammed with protests of the bill, most from outside his state.

"You have to give them credit: The phone calls, the faxes, the people who show up at town halls and meetings — you have to say NumbersUSA is behind a fair amount of that," said Frank Sharry, director of the National Immigration Forum, a nonprofit group that advocates for immigrants.

Sharry acknowledged NumbersUSA's influence on lawmakers, pointing to Georgia's two Republican senators, Johnny Isakson and Saxby Chambliss. The two, who helped write the immigration bill, were immediately in NumbersUSA's crosshairs. Both have withdrawn their support, saying the bill fails to provide adequate border security.

But Sharry argued that NumbersUSA had yet to make a dent at the ballot box.

"Their weakness shows up on election day," he said. "Why is Trent Lott, who is getting hammered with letters and faxes and talk radio, getting stronger in favor of the bill instead of weaker? He knows that for all their bluster, there's no real threat."

NumbersUSA says population growth is damaging the country — creating urban sprawl, snarling commuter roads, straining schools and hospitals, and diminishing natural resources. They say immigration propels much of this growth and should be restricted.

The United States currently issues about 1 million visas annually for legal permanent residency. The group wants that number to drop to the early 20th century level, about 250,000.

The Senate bill would not limit total numbers of legal immigration, and NumbersUSA doubts it would effectively stop illegal entries.

Beck started NumbersUSA in 1997, after working as the Washington editor of the Social Contract, a conservative magazine. Today his group operates out of sleek offices in Rosslyn, Va., across the Potomac River from Washington.

Critics have cited ties between NumbersUSA and controversial right-wing millionaire John Tanton, publisher of the Social Contract and founder of the Federation for American Immigration Reform. That group in the past accepted funding from the Pioneer Fund, a conservative organization that also has funded research into eugenics.

Beck said NumbersUSA started out under Tanton's umbrella but stopped receiving funding from him a few years ago.

Voluble, with graying sandy hair that curls around his ears, 58-year-old Beck calls himself "a frenetic person." He regularly works past midnight and spent Saturday making lengthy phone calls and shooting off e-mail alerts. He approaches running NumbersUSA with "all the neuroses of a small-business owner."

These should be comforting times. Beck's e-mail list reaches 1.5 million subscribers, but he also has a corps of activist members — those who send faxes and letters and get involved in other ways, such as delivering petitions, like Wade in Mississippi.

Beck monitors how many are active as an indicator of the organization's clout.

In January 2001, the group had 1,679 activist members. This January, the group had close to 244,000; by Friday, that number reached the current 419,000 activists.

In May alone, they sent senators 750,000 faxes. They transmitted more than 100,000 on a single day in June when a crucial vote was scheduled.

People began joining after the 2001 terrorist attacks. Other membership factors that Beck cites include talk radio, the massive immigration marches in 2006, the October announcement that the U.S. population had reached 300 million, and crucially, the Internet.

But he credits one primary reason for his group's exploding membership: President Bush.

In January 2004, Bush announced his intent to overhaul immigration and offer legal status to the estimated 12 million illegal immigrants. That year, activist membership jumped 326%, to 48,000. "Before then, we had never envisioned ourselves as being a mass movement," Beck said. "George Bush changed our vision."

Now, every time the president raises the issue, membership ticks up. "He makes people angry," Beck said.

Recently, members could fax senators on a list of the "Flippin' Fifteen." These are lawmakers who opposed the Senate bill earlier, but now are under pressure to reverse that stance. The list includes Sen. Barbara Boxer (D-Calif.), who expressed concerns about the bill's fairness to immigrants.

Of California's 56,987 NumbersUSA members, 28,525 are considered activists.

Beck thinks the group's chances are good of influencing lawmakers like Boxer and the final outcome on the Senate bill this week.

Even though many elected officials, lobbyists and pro-immigration groups support the legislation, Beck points to news reports from January when the Democrats, newly in control of Congress, were pledging alongside Bush to pass an immigration bill.

"We're in June and the fact is it still hasn't gone through the Senate," said Beck. "I think we still have a good chance to beat it in the Senate. I think that's the impact of the grass roots."