US Senator Richard Lugar, a senior Republican and a reliable vote for President George W. Bush on the war, said that Bush's Iraq strategy was not working and that the US should downsize the military's role.
The unusually blunt assessment deals a strong political blow to Bush, who has relied heavily on Republican support to stave off anti-war legislation.
It also comes as a surprise. Most Republicans have said they were willing to wait until September to see if Bush's recently ordered buildup of troops in Iraq was working.
"In my judgment, the costs and risks of continuing down the current path outweigh the potential benefits that might be achieved," Lugar said in a Senate floor speech on Monday. "Persisting indefinitely with the surge strategy will delay policy adjustments that have a better chance of protecting our vital interests over the long term."
Only a few Republicans have broken ranks and called for a change in course or embraced Democratic proposals ordering troops home by a certain date. As the top Republican and former chairman of the Senate Foreign Relations Committee, Lugar's critique could provide political cover for more Republicans wanting to challenge Bush on the war.
Lugar's spokesman Andy Fisher said the senator had wanted to express his concerns publicly before Bush reviews his Iraq strategy in September.
"They've known his position on this for quite a while," Fisher said of the White House.
However, Fisher said that the speech does not necessarily mean Lugar would switch his vote on the war or embrace Democratic measures setting a deadline for troop withdrawals.
In January, Lugar had voted against a resolution opposing the troop buildup, contending that the nonbinding measure would have no practical effect.
In the spring, he voted against a Democratic bill that would have triggered troop withdrawals by Oct. 1 with the goal of completing the pullout in six months.
Next month, Senate Majority Leader Harry Reid plans to force votes on several anti-war proposals as amendments to a 2008 defense policy bill.
Members will decide whether to cut off money for combat, demand that troop withdrawals start in four months, restrict the length of combat tours and rescind Congress' 2002 authorization of the Iraqi invasion.