WASHINGTON  So powerful was Representative John P. Murtha at one time that he used to put up billboards in his Western Pennsylvania district declaring that “the P is for Power.” Few in Congress dared disagree: he doled out or withheld billions in federal money each year for lawmakers’ pet projects, better known as earmarks.

Now, however, a string of federal criminal investigations of contractors or lobbyists close to Mr. Murtha, the top Democrat on the defense appropriations subcommittee, are threatening to undermine his backroom clout.

In the weeks since the news that prosecutors had raided the offices of the PMA Group  a lobbying firm founded by a former Murtha associate that became a gateway to his office and his biggest source of campaign money  about two dozen rank-and-file Democrats have risked his wrath by calling for a House ethics investigation of the matter. One Democrat has even foresworn seeking earmarks for the military contractors in his district because of ethical concerns about the process.

In a private meeting with the chairman of the House appropriations committee, Mr. Murtha, the unofficial leader of the “old bulls” who oversee the subcommittees, was forced to accept a series of new restrictions on his authority to grant earmarks, Democratic aides briefed on the meeting said. In previous weeks he had already acquiesced to another steep cut in the volume of earmarks he dispenses, down by half this year from a few years ago. He had also submitted to new disclosure requirements, including public hearings, that cramp his ability to cut last-minute deals.