Responsibility to troops
Bush said at a news conference on Wednesday that there is no doubt the Iranian government is providing armour-piercing weapons to kill American troops in Iraq.
But he backed away from claims by senior US military officials in Baghdad that high-ranking Iranian officials were responsible.
Critics of the Bush administration have accused the president of looking for a pretence to attack Iran.
In a speech on Thursday, Bush said he expects congress to live up to its promise to "support the troops".
He said: "We have a responsibility, Republicans and Democrats have a responsibility, to give our troops the resources they need to do their job and the flexibility they need to prevail."
In the third day of the lower house debate on the war, Republican combat veterans spoke out against the Democratic resolution.
"The enemy wants our men and women in uniform to think their congress doesn't care about them"
Sam Johnson, Republican representative
Sam Johnson, a Republican representative who was a prisoner of war during Vietnam, said: "The enemy wants our men and women in uniform to think their congress doesn't care about them.
"We must learn from our mistakes. We cannot leave a job undone like we left in Korea, like we left in Vietnam, like we left in Somalia."
Duncan Hunter, the senior California Republican representative on the House Armed Services Committee, called the political manoeuvring by Democrats to micromanage the war "extremely dangerous".
He said: "It could stop reinforcements from arriving in time to stop major casualties in any of a number of scenarios."
Democrats face opposition in the senate as well.
Lindsey Graham, a Republican senator, said: "I will do everything in my power to ensure the house resolution dies an inglorious death in the senate."