WASHINGTON — A new national intelligence estimate on Iraq is providing momentum for a drive by Democrats in Congress to end the war, and providing Senator McCain an opportunity to accuse Senator Clinton of flip-flopping.

Yesterday, America’s director of national intelligence released an Iraq assessment that concluded Iraq’s government was in a “precarious” state and that there was little chance for national political reconciliation in the next six to 12 months.

At the same time, the new assessment also acknowledges that local “bottom-up” political reconciliation is occurring in the provinces where Sunni sheiks are taking up arms against Al Qaeda and fighting alongside American GIs. The report predicts that security will modestly improve under the current strategy in the coming months and warns that changing the current military mission would “erode security gains achieved thus far.”

The new assessment concludes that seeking a replacement for Prime Minister Maliki would likely “paralyze the government,” a rejection of statements made earlier this week by the chairman of the Senate Armed Services Committee, Carl Levin, a Democrat of Michigan, who said he hoped the Iraqi parliament would replace the Iraqi premier.

The fate of Iraq and the Bush presidency depends on the assessment delivered to Congress next month by General David Petraeus and America’s ambassador in Iraq, Ryan Crocker. In the last month, a growing number of centrist Democrats have conceded that, militarily, General Petraeus is gaining ground. At the same time, the intelligence assessment’s bleak view of the prospect for any political compact between Iraq’s Sunnis and Shiites will also lead many war critics in Congress to press for withdrawal.

The Senate majority leader, Harold Reid, a Democrat of Nevada, yesterday said the new estimate “confirmed that the Iraqi government is failing to achieve meaningful political reconciliation, the stated goal of the president’s current Iraq policy, while our troops are forced to police a civil war.” He concluded his statement to the press by calling for a change in strategy. In July, Mr. Reid supported a firm withdrawal date of April 2008 as an amendment to the defense authorization bill. He lacked the votes to override a presidential veto or stop a Republican filibuster.

The top Republican on the Senate Armed Services Committee, John Warner of Virginia, told PBS yesterday that he suggested to the president that he pick a date for withdrawal before September 15, the date by when President Bush is required by law to report to the public and Congress any changes in Iraq strategy he may deem appropriate. Mr. Warner yesterday said the new intelligence assessment confirmed much of what he had been thinking for the last three months. In the interview, the senator said he told the president, ‘It’s up to you, Mr. President, in consultation with your military commanders.'”

It’s unclear Mr. Warner would actually vote for a hard withdrawal date. He said, “Now, they’re perfectly willing to reject it. If that’s what they want to do, reject it. But at least I have spoken out with clarity and cleared my own conscience.”

Jockeying over the war has also led Mrs. Clinton to disavow her acknowledgement that the American troop surge was making progress. She told an audience Monday at the annual convention of Veterans of Foreign Wars that the surge was “working,” but that it was “too late.” On Wednesday, after coming under fire from a rival Democratic presidential candidate, John Edwards, for detracting from the push to end the war, Mrs. Clinton said, “The surge was designed to give the Iraqi government time to take steps to ensure a political solution. … It has failed.”

Yesterday, Senator McCain, a Republican from Arizona who is running for president, criticized Mrs. Clinton, saying, “The fact that the New York senator can reverse her position on an issue of grave importance to our national security in a few days sends the wrong signal to our enemies in Iraq and our own troops on the ground.”

The intelligence report previews a line of argument that will emerge next month for those lawmakers who will remain anti-war after General Petraeus’s report. Ambassador Crocker himself this week said political reconciliation prospects were “disappointing,” despite a new coalition of Shiite and Kurdish political parties.

The new intelligence estimate acknowledges that the success of the federal security forces to bring violence down in Baghdad depends in part on the commitment to the war from Washington. “Perceptions that the Coalition is withdrawing probably will encourage factions anticipating a power vacuum to seek local security solutions that could intensify sectarian violence and intra-sectarian competition,” it says. “At the same time, fearing a Coalition withdrawal, some tribal elements and Sunni groups probably will continue to seek accommodation with the Coalition to strengthen themselves for a post-Coalition security environment.”

The classified version of the national intelligence estimate acknowledges that the Mujahadin Shura Council, an Al Qaeda-dominated body attached to the group’s shadow Islamic state, has overtaken the Sunni insurgency. In an earlier national intelligence estimate on Iraq, released in February, the question of Al Qaeda’s role in the overall Sunni insurgency had been disputed, and those who believed al Qaeda had taken over the insurgency were relegated to voicing their dissent in a footnote.