WASHINGTON – President Bush is set to ask the country for more time to let success bloom in Iraq even as the assassination of a Sunni sheik who revolted against al-Qaida sent a chilling reminder of the instability there.

The president’s pitch for more time _ which has aggravated Democrats as well as many rank-and-file Republicans _ is scheduled for a televised address at 9 p.m. EDT.

Not willing to wait, Senate Democrats are discussing legislation to limit the mission of U.S. forces in Iraq. By limiting troops to training Iraq’s military and police, protecting U.S. assets and fighting terrorists while not setting a deadline to end the war, they hope to attract enough Republicans to bring such a proposal to a vote.

The day after his televised address from the Oval Office _ expected to last 18 minutes _ Bush is expected to reinforce the message with remarks from a Marine base in Quantico, Va., just outside Washington, and with the White House’s release of an Iraq status report required by Congress.

However, the assassination Thursday of Sunni sheik Abdul-Sattar Abu Risha, who took on al-Qaida, was a setback for U.S. efforts, serving notice of the danger facing people who cooperate with coalition forces.

The White House called his death an “unfortunate and outrageous act” and said it believed al-Qaida was responsible.

“This is a sheik who was one of the first to come forward to want to work with the United States to repel al-Qaida from al-Anbar Province,” said White House spokeswoman Dana Perino. “Remember, al-Qaida was killing some of the sheiks’ children and put them in a cooler to deliver to the sheiks.”

“This is the kind of enemy we’re dealing with,” she said.

Abu-Risha was the leader of the Anbar Salvation Council, also known as the Anbar Awakening _ an alliance of clans backing the Iraqi government and U.S. forces _ and was among a group of tribal leaders who met with Bush earlier this month at al-Asad Air Base in Anbar province.

His assassination is “an indication of, while there has been great progress, dramatic progress, in some areas of Iraq, there is still some work to be done,” said State Department spokesman Sean McCormack.

“This is a tragic loss,” Gen. David Petraeus said in a statement released by his spokesman. “It’s a terrible loss for Anbar province and all of Iraq. It shows how significant his importance was and it shows al-Qaida Iraq remains a very dangerous and barbaric enemy. He was an organizing force that did help organize alliances and did help keep the various tribes together.”

As part of the administration’s public relations push to sell the Iraq policy, Vice President Dick Cheney planned to travel to the Gerald R. Ford Presidential Museum in Michigan and MacDill Air Force Base in Florida. Iraq also was chosen as the topic for Bush’s weekly Saturday radio address, and administration officials were being offered to television networks for Sunday news show appearances.

The full-throttle effort to get out Bush’s message on Iraq reflects the high stakes for a president who lost his popularity and his party’s control of Congress in large measure over the war and yet ordered 21,500 additional combat troops there in January to try to bring calm and give his goal of a stable, self-sustaining Iraq a chance. An additional 8,000 support troops soon followed.

Gradually phasing out that force escalation by next July _ Bush’s plan to be announced Thursday night _ would still leave about 135,000 U.S. troops in Iraq. Democrats against the war were not at all satisfied.

“It creates and provides an illusion of change in an effort to take the wind out of the sails of those of us who want to truly change course in Iraq,” said Sen. Carl Levin, D-Mich., chairman of the Armed Services Committee.

Bush’s latest approach matches Petraeus’ recommendations for the force posture in Iraq. What Bush is endorsing, however, represents only a slight hastening of the already scheduled end of the so-called surge.

Planned troop rotations call for the first of the additional troops sent this year to come back starting next spring, with all of them home by the end of next summer. That could change only if Bush and his team extend tour lengths or shorten home leave for replacements.

The plan Petraeus announced this week would instead have the first of the additional troops returning home this month, a 2,000-member Marine unit, followed by the mid-December departure of an Army brigade numbering 3,500 to 4,000 soldiers. The other four combat brigades would leave by July 2008 under an unspecified timetable. Also, Petraeus was not precise about whether all of the 8,000 support troops sent with those extra combat forces would be withdrawn by July.

The general cautioned against any talk before March of whether it would be wise to go below the pre-surge level of 135,000, and Bush is expected to endorse that idea. The president also will say that even ending the surge will depend on continued security gains and no unforeseen events that change the dynamic.

Bush was expected to cite progress but also ready to talk candidly about the fact that few of the 18 benchmarks that Congress and the White House set to measure progress have been met. But the administration has sought to deflate the importance of those benchmarks, which are required to be the focus of the Friday’s report but which the White House now regards as offering an unrealistic or incomplete look at the situation.

Bush rehearsed his speech Wednesday in the White House theater and talked by phone with Petraeus and U.S. Ambassador Ryan Crocker, thanking them for their service. White House aides also were reaching out to key foreign and domestic leaders to preview the broad outlines of the speech, but not its details.

Leading Senate Democrats, meanwhile, huddled in Senate Majority Leader Harry Reid’s office to discuss how to regain momentum in the debate.