Summary: Since “suffering, misery, and dishonor” will ensue no matter what the United States does next in Iraq, it's difficult to decide what our policy should be. With respect to Iran, however, it's a no-brainer:

War with Iran would be a catastrophe that would make us look back fondly on the minor inconvenience of being bogged down in Iraq. While the Congress flounders about what, exactly, it can do about Iraq, it can do something useful, while it still matters, in making clear that it will authorize no money and provide no endorsement for military action against Iran.

The argument that the Bush administration is simply rattling the saber in order to make its diplomatic efforts towards Iran more effective fails, because

no one can any longer trust the Administration to recognize and defend America's rational self-interest -- not when the President says he will carry out a policy even if opposed by everyone except his wife and dog, not when the Vice President refuses to concede any mistake or misjudgment in the handling of Iraq.

Threats don't really promote diplomacy when they are delivered by advocates of preventive war.