Further threatening the Bush administration's stance on Iraq policy, two top Senate Republicans introduced legislation on Friday that would require the president to come up with a plan that would drastically reduce the U.S. military presence there.

Sens. John Warner and Richard Lugar teamed together to draft the bill, which is being offered in the form of an amendment to the defense authorization bill being considered in the Senate.

The legislation also came as the Pentagon acknowledged that a decreasing number of Iraqi army battalions are able to operate independently of U.S. troops. And on Thursday, the administration issued a mix assessment on benchmarks being met by the Iraq government.

"Given continuing high levels of violence in Iraq and few manifestations of political compromise among Iraq's factions, the optimal outcome in Iraq of a unified, pluralist, democratic government that is able to police itself, protect its borders, and achieve economic development is not likely to be achieved in the near future," the Warner-Lugar proposal said.

• Click here to see the entire bill, Amendment No. 2208 (.pdf).

President Bush and members of his administration sought Thursday and Friday to rally support for his troop surge plan, which he put in place in January. The plan called for 30,000 more combat troops to head to the war-torn country to provide more security so the Iraqi government could gain a better foothold.

Since then, Congress has mandated Bush to provide two reports on progress in Iraq — the first of which was released Thursday. The next report is due in September, but Bush has called on lawmakers not to make up their mind on his plan until then. And among several attempts to bring troops home, the House on Thursday passed a bill — which the president has said he will veto — that would set April as a deadline for troop withdrawals.

Warner and Lugar's bill is being seen as a more serious threat to Bush's policy, however. Warner, of Virginia, and Lugar, of Indiana, are the top Republicans on their respective committees — Warner on the Senate Armed Services Committee, and Lugar on the Senate Foreign Relations Committee — and both are highly respected as much for what they say as what they don't.

Warner, a former Navy secretary, raised eyebrows last year when he noted that Iraq seemed to be "drifting sideways." This month, Lugar — who has been supportive of the war thus far — startled other Republicans when he called for a new policy in Iraq. They have been reluctant to issue criticism over the president's war policies.

In addition to calling on the president to plan troop reductions, the bill also is noteworthy for its statement regarding the original Iraq war authorization in 2002, which the bill calls "obsolete and needs revision."

The bill says: "American military and diplomatic strategy in Iraq must adjust to the reality that sectarian factionalism is not likely to abate anytime soon and probably cannot be controlled from the top."

Accordingly, Warner and Lugar say Bush must draft a plan for U.S. troops that would keep them from "policing the civil strife or sectarian violence in Iraq" and focus them instead on protecting Iraq's borders, targeting terrorists and defending U.S. assets.

The amendment requires the president to produce a host of contingency plans for the war, including redeployment. It also endorses a troop drawdown to a limited mission that was outlined by the Iraq Study Group

The Warner-Lugar plan says the new plans must be executable by Dec. 31, 2007, although it does not say they must be complete by then.

The amendment also require an updated National Intelligence Estimate, as well.

"Like all members of the Senate, I have great respect for the Constitutional separation of powers between the executive and legislative branches and the authorities granted to each in that document. My views are matters of conscience and reflect what I believe to be in the best interests of the country and our men and women in uniform and their families, for whom I have the deepest admiration," Warner said in a statement released by his office.

"It is my sincere hope that this amendment provides a basis for a bipartisan consensus," he added.

Lugar said: "Although I do not doubt the evaluation of U.S. military leaders in Iraq that there has been military progress, I believe there is strong evidence that the Iraqi government and political system will not achieve necessary political accommodations in a short time frame. I believe that continuing with the surge delays policy changes that have a far better chance of protecting our vital interests in the region over a sustained period."

At the Pentagon, meanwhile, Marine Gen. Peter Pace, chairman of the Joint Chiefs of Staff, told reporters that the number of battle-ready Iraqi battalions able to fight on their own has dropped to a half-dozen from 10 in recent months despite heightened American training efforts.

Without providing numbers, the White House had acknowledged in its report to Congress Thursday that not enough progress was being made in training Iraqi security forces — an issue that determines to a large extent when the United States may be able to reduce its forces there.

Pace, however, also said the readiness of the Iraqi fighting units was not an issue to be "overly concerned" about because the problem is partly attributable to the fact that the Iraq units are out operating in the field.

Appearing at a news conference with Defense Secretary Robert Gates, Pace said that "as units operate in the field, they have casualties, they consume vehicles and equipment."

FOX News' Trish Turner and The Associated Press contributed to this report.