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.