Stephen Crowley/The New York Times

Senators Mel Martinez, center, Republica of Florida, said he had received a threatening letter related to the immigration bill that he planned to turn over to the authorities. Other senator report similar experiences.

WASHINGTON, June 27 — The threat came in the weekend mail.

The recipient was Senator Mel Martinez, Republican of Florida, who has been a leading advocate of the proposed legislation for changing the immigration system. His offices in Washington and across Florida have received thousands of angry messages in recent weeks, but nothing as alarming as that letter he received at his home.

“I’ll turn it over to Capitol police, and we’ll go from there,” said Mr. Martinez, who declined to elaborate on the nature of the threat.

On the eve of a crucial vote on the immigration bill, the Capitol Hill switchboard was deluged again Wednesday as thousands of citizens called their members of Congress — and, perhaps, someone else’s — to weigh in. Not since the impeachment of President Bill Clinton, several Senate aides said, have the lines been so jammed by a single issue.

Republicans who support the immigration bill are facing unusually intense opposition from conservative groups fighting it. This is among the first times, several of them said, that they have felt the full brunt of an advocacy machine built around conservative talk radio and cable television programs that have long buttressed Republican efforts to defeat Democrats and their policies.

While the majority of the telephone calls and faxes, letters and e-mail messages have been civil, aides to several senators said, the correspondence has taken a menacing tone in several cases.

Senator Richard Burr, a North Carolina Republican who is undecided on the final immigration bill, said his office received a telephone call recently that “made a threat about knowing where I lived.” Mr. Burr passed it along to the authorities. “There were enough specifics to raise some alarm bells,” he said.

Senator Lindsey Graham, a South Carolina Republican who is one of the architects of the immigration overhaul, said he also had received threats in telephone calls and letters to his office. Mr. Graham said several other senators had told him privately that they also received similar messages.

“There’s racism in this debate,” Mr. Graham said. “Nobody likes to talk about it, but a very small percentage of people involved in this debate really have racial and bigoted remarks. The tone that we create around these debates, whether it be rhetoric in a union hall or rhetoric on talk radio, it can take people who are on the fence and push them over emotionally.”

The immigration legislation, a priority of President Bush’s, has divided the Republican Party. For the past month, no other issue has been debated as passionately among conservatives as this bill, which calls for the most sweeping changes to immigration law in two decades.

At the heart of the opposition rests conservative hosts on talk radio and cable television, which often are a muscular if untamed piece of the Republican message machine.

Several senators said Wednesday that they did not care to be identified speaking critically of the broadcasters, fearing the same conservative backlash that befell Senator Trent Lott, a Mississippi Republican, this month when he declared: “Talk radio is running America. We have to deal with that problem.”

Organizations that have mobilized tens of thousands of people to speak against the immigration legislation said they did not advocate threats. A leading group, Grassfire .org, said that its members had made 250,000 contacts this week with offices of United States senators.

Senator John W. Warner, Republican of Virginia, said he had never seen an issue stir such a public response. “In my 29 years, I’ve experienced all the events in that period of time,” he said, “but this is clearly the high-water mark.”

Since Mr. Warner arrived in the Senate, technology advances have made it easier to deliver more messages to members of Congress. Many e-mail messages sent to the Senate are copied to multiple offices, including one that was forwarded to the authorities this week. Referring to supporters of the bill, it closed with the line: “They need to be taken out by ANY MEANS.”

Sgt. Kimberly Schneider, a spokeswoman for the United States Capitol Police, said it was the department’s policy not to discuss potential threats against lawmakers.

As Mr. Graham walked back to his office on Wednesday, he said he doubted that senators would be deterred by any threats. “I’m sure a lot of the people who have taken a high-profile position on this have been threatened, but what are you going to do?” he said. “You saw what happened to Senator Daschle.”

Mr. Graham was referring to Tom Daschle, the former Democratic majority leader from South Dakota, whose office received a mailing of anthrax in 2001. The case remains unsolved.

“One of the requirements of public service in modern America is dealing with a few voices that are full of hate,” Mr. Graham said. “And our discourse and the way we politic, the way we engage each other, brings that out.”

Copyright 2007 The New York Times Company. Reprinted from The New York Times, National, of Thursday, June 28, 2007.