But those same insiders said that in retrospect Mr. Cheney’s power was at its peak in 2003 and 2004, before Iraq’s insurgency flared, before the abuses at Abu Ghraib were revealed, before the setbacks in Congress and at the Supreme Court.

Without the help of his closest adviser, I. Lewis Libby Jr., who resigned last fall after his indictment in the Central Intelligence Agency leak case, Mr. Cheney has lost the early warning radar that gave him and his staff such command over the federal bureaucracy. Administration insiders said Mr. Cheney and his aides were now having to fight to maintain positions that a few years ago he would have won handily.

“During the first term, Cheney was considered one of the smartest guys in the administration,” said Representative Ray LaHood, Republican of Illinois. “But his influence has been diminished because of the Scooter Libby thing and because the war in Iraq has not gone well.”

In his second term, Mr. Bush has grown less dependent on Mr. Cheney for information, current and former officials said. When Joshua B. Bolten became the White House chief of staff earlier this year, he told associates that he wanted to make sure the president heard from more voices. “My impression is that there are a lot more data points or gathering points now,” said Senator John Thune, Republican of South Dakota.

For instance, Mr. Bush has turned to another Washington insider, James A. Baker III, who served Mr. Bush’s father as secretary of state, for help as the co-chairman of an outside group developing options for dealing with Iraq. One group member said, “You get the sense that the president now realizes, perhaps a little late, that he needs Baker to find him an exit door.”

Mr. Cheney told NBC News in May that his influence with Mr. Bush was unchanged. “I feel like he gives me the access that I need to be able to do my job,” Mr. Cheney said, adding later: “I give him the best advice I can. He doesn’t always agree. Sometimes he does, sometimes he doesn’t.”

Challenges in Congress

Perhaps nowhere has Mr. Cheney’s shifting influence been more visible than on Capitol Hill, where the vice president’s ability to win his way without challenge — a luxury he enjoyed through much of the first term — has evaporated.