The newspaper says its examination of an Obama campaign Web site shows thousands of one-time backers of the Illinois U.S. senator are now blasting him for his stance on upcoming terrorist surveillance legislation.

Obama modified his opposition to the bill once a compromise emerged giving legal immunity to U.S. telecommunications companies that allowed the Bush administration to wiretap suspect telephone calls without a warrant. Obama opposed immunity, but once it was included, he decided to back the bill anyway, the Tribune reported.

His decision has triggered furious messages on the Web site, showing, perhaps for the first time, how a political campaign that uses the Internet extensively for fundraising and organizing can also be stung by it, the newspaper said.

"This campaign has an extraordinary group of committed supporters, and we greatly appreciate their willingness to share their time and ideas with us," Obama spokesman Tommy Vietor said. "We believe that an open dialogue is an important part of any campaign."