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...