WASHINGTON (AP) – The top American commander in Iraq told a Senate panel Thursday that improving security in Baghdad would take fewer than half as many extra troops as President Bush has chosen to commit.

Testifying before the Senate Armed Services Committee on his nomination to be Army chief of staff, Gen. George Casey said he had asked for two additional Army brigades, based on recommendations of his subordinate commanders. Mr. Bush announced Jan. 10 that he would send five extra brigades as part of a buildup that would total 21,500 soldiers and Marines.

Asked by Senator Warner why he had not requested the full five extra brigades that Mr. Bush is sending, Mr. Casey said, “I did not want to bring one more American soldier into Iraq than was necessary to accomplish the mission.”

With many in Congress opposing or skeptical of Bush’s troop buildup, Mr. Casey did not say he opposed the president’s decision. He said the full complement of five brigades would give U.S. commanders in Iraq additional, useful flexibility.

Even so, Mr. Casey’s comments seemed put distance between his views and those of Bush and some lawmakers like Senator McCain who have questioned whether Mr. Bush’s troop increase will be enough.

Mr. McCain criticized Mr. Casey for what he called misjudgments about the prospects for progress toward stabilizing Iraq during his tenure. Mr. McCain said he has “strong reservations” about Mr. Casey’s nomination to become Army chief of staff.

“While I don’t in any way question your honor, your patriotism or your service to our country, I do question some of the decisions, the judgments you’ve made over the past two and a half years,” Mr. McCain, top Republican on the committee, told Mr. Casey. “During that time things have gotten markedly and progressively worse.”

Mr. McCain asked Mr. Casey whether he thought the mission in Baghdad could be accomplished with fewer than five extra brigades.

`I believe that the job in Baghdad, as it’s designed now, can be done with less than that,” Mr. Casey said. “But having the flexibility to have the other three brigades on a deployment cycle gives us and gives General Petraeus great flexibility,” he added, referring to his designated successor, Lt. Gen. David Petraeus.

“It allows him to make assessments on whether the plan is working or not and to either reinforce success, maintain momentum, or put more forces in a place where the plans are not working,” Mr. Casey said.

Mr. Casey described the situation in Baghdad as “bad,” and said the U.S. strategy was not succeeding in three areas of the country: the provinces of Anbar and Diyala, as well as in Baghdad.

The proposed Senate resolution opposing Bush’s troop buildup is likely to pose a threat to the White House because of its potential appeal to Republicans who have grown tired of the nearly four-year war and want a chance to express their concerns. The White House has been hoping to avoid an overwhelming congressional vote criticizing Bush’s handling of the war.

Democratic Sen. Carl Levin and Republican Sen. John Warner agreed Wednesday to offer a resolution that would oppose Mr. Bush’s decision to send 21,500 more troops into Iraq but protect funding for them.

One liberal, Senator Feingold issued a statement criticizing the nonbinding measure as weak, and said it “misunderstands the situation in Iraq and shortchanges our national security interests. The resolution rejects redeploying U.S. troops and supports moving a misguided military strategy from one part of Iraq to another.”

At Thursday’s hearing, Mr. Warner pressed Mr. Casey on why so many additional U.S. forces should be added to the fight in Baghdad.

“Why are we not putting greater emphasis on utilization of Iraqi forces and less on the U.S. GI being put into that cauldron of terror…?” Mr. Warner asked, adding that he hoped that at least some of the 21,500 extra troops will not be sent. The last of the five extra brigades is scheduled to go in May.

Mr. Casey said that Iraqis are taking more of a lead role, but are not yet ready to fight without American support.

At the White House, presidential spokesman Tony Snow was asked whether the administration is resigned to seeing Congress approve some form of resolution opposing a troop buildup.

“We’re not resigned to anything,” Mr. Snow said. “We’re determined to move forward toward success in Iraq. Again, rather than trying to pull out the crystal ball, let’s just see what happens on the Hill.”

He said the White House is keeping an eye on the debate, but “the president’s also not in the business of writing resolutions for a separate and coequal branch of government.”

In his opening statement to the Senate Armed Services Committee, Mr. Casey defended his record as the top commander in Iraq, saying he remained true to his original commitment to request the number of troops he thought he needed to accomplish his mission.

Asked his view of Mr. Bush’s new strategy, Mr. Casey said, “I believe it can work.”

He said success in Baghdad could be achieved with fewer than five extra brigades, but he added that this plan will give “great flexibility” to his successor.

Mr. Casey also spoke optimistically of the situation in Iraq.

“The struggle in Iraq is winnable,” Mr. Casey said, but will take patience and will.

After asserting last week that “I’m the decision-maker” about troop levels in Iraq, Bush acknowledged that Congress has the power to cap force levels and put conditions on where soldiers are deployed.

“They can say, `We won’t fund,’ ” he told The Wall Street Journal. “That is a constitutional authority of Congress. … They have the right to try to use the power of the purse to determine policy.” As for Congress having a voice on where troops go, Bush said, “They put conditions on funds all the time.”

Mr. McCain read to Mr. Casey a number of quotations from public statements he made during his time in Baghdad and pressed him to acknowledge failure.

“I do not believe that the current policy has failed,” Mr. Casey said.

Mr. McCain did not state flatly that he would oppose Mr. Casey’s nomination to be Army chief of staff, replacing Gen. Peter Schoomaker. But he spoke harshly of Mr. Casey’s record in Baghdad.

“I question seriously the judgment that was employed in your execution of your responsibilities in Iraq,” he said. “And we have paid a very, very heavy price in American blood and treasure because of what is now agreed to by literally everyone as a failed policy.”