Washington - Senator Barack Obama's decision to support legislation granting legal immunity to telecommunications companies that cooperated with the Bush administration's program of wiretapping without warrants has led to an intense backlash among some of his most ardent supporters.

    Thousands of them are now using the same grass-roots organizing tools previously mastered by the Obama campaign to organize a protest against his decision.

    In recent days, more than 7,000 Obama supporters have organized on a social networking site on Mr. Obama's own campaign Web site. They are calling on Mr. Obama to reverse his decision to endorse legislation supported by President Bush to expand the government's domestic spying powers while also providing legal protection to the telecommunication companies that worked with the National Security Agency's domestic wiretapping program after the Sept. 11 attacks.

    During the Democratic primary campaign, Mr. Obama vowed to fight such legislation to update the Foreign Intelligence Surveillance Act, or FISA. But he has switched positions, and now supports a compromise hammered out between the White House and the Democratic Congressional leadership. The bill is expected to come to a vote on the Senate floor next Tuesday. That decision, one of a number made by Mr. Obama in recent weeks intended to position him toward the political center as the general election campaign heats up, has brought him into serious conflict for the first time with liberal bloggers and commentators and his young supporters.

    Many of them have seen the issue of granting immunity to the telecommunications companies as a test of principle in their opposition to Mr. Bush's surveillance program.

    "I don't think there has been another instance where, in meaningful numbers, his supporters have opposed him like this," said Glenn Greenwald, a Salon.com writer who opposes Mr. Obama's new position. "For him to suddenly turn around and endorse this proposal is really a betrayal of what so many of his supporters believed he believed in."

    Jane Hamsher, a liberal blogger who also opposes immunity for the phone companies, said she had been flooded with messages from Obama supporters frustrated with his new stance.

    "The opposition to Obama's position among his supporters is very widespread," said Ms. Hamsher, founder of the Web site firedoglake.com. "His promise to filibuster earlier in the year, and the decision to switch on that is seen as a real character problem. I know people who are really very big Obama supporters are very disillusioned."

    One supporter, Robert Arellano, expressed his anger on the Obama site.

    "I have watched your campaign with genuine enthusiasm," Mr. Arellano wrote, "and I have given you money. For the first time in my life, I have sensed the presence of a presidential candidate who might actually bring some meaningful change to the corrupt cesspool of national politics. But your about-face on the FISA bill genuinely angers and alarms me."

    For now, the campaign is trying to put a positive spin on the new FISA fight among its supporters.

    "The fact that there is an open forum on BarackObama.com where supporters can say whether they agree or disagree speaks to a strength of our campaign," said Bill Burton, a campaign spokesman.

    Several activists and bloggers predicted that Mr. Obama's move toward the center on some issues could sharply reduce the intensity of support he has enjoyed from liberal activists. Such enthusiasm helped power his effort to secure the Democratic nomination, and it has been one of his campaign's most important tools for fund-raising and organizing around the country.

    Markos Moulitsas, a liberal blogger and founder of the Daily Kos Web site, said he had decided to cut back on the amount of money he would contribute to the Obama campaign because of the FISA reversal.

    "I will continue to support him," Mr. Moulitsas said in an interview. "But I was going to write him a check, and I decided I would rather put that money with Democrats who will uphold the Constitution."

    Greg Craig, a Washington lawyer who advises the Obama campaign, said Tuesday in an interview that Mr. Obama had decided to support the compromise FISA legislation only after concluding it was the best deal possible.

    "This was a deliberative process, and not something that was shooting from the hip," Mr. Craig said. "Obviously, there was an element of what's possible here. But he concluded that with FISA expiring, that it was better to get a compromise than letting the law expire."