In Friday’s presidential debate on foreign policy (assuming the show still goes on), we can be sure that Barack Obama will hit John McCain hard for supporting what Obama has called a “dumb war” in Iraq. But in doing so, Obama has at least one major handicap to overcome: his running mate.

In October 2002, Sen. Biden voted to authorize the Iraq war. “I made a mistake,” he now says — he had “vastly underestimated” how incompetent the Bush administration would be in prosecuting the war.

So has Biden changed his position on Iraq? Not really. In October 2002, when the congressional vote was held, Biden, like most members of Congress, was in favor of avoiding accountability and punting the question of war or peace to the president. And Biden remains firmly in favor of avoiding accountability for Iraq today. That tells us something about Joe Biden’s judgment and integrity. More importantly, it tells us a lot about the health of Congress as a political institution, and about the erosion of Congress’s power to declare war.

As James Madison put it, “In no part of the Constitution is more wisdom to be found, than in the clause which confides the question of war or peace to the legislature, and not to the executive department.” But Biden, and a majority of senators and House members, ignored that wisdom in 2002 and voted for a use-of-force resolution that handed a “blank check” to the president, as Sen. Robert Byrd rightly observed at the time. True, the resolution features some boilerplate about exhausting other options before using force, and prominent lawmakers have used that language to suggest they didn’t “really” vote for war. But the operative clause of the resolution — “The President is authorized to use the Armed Forces of the United States as he determines to be necessary and appropriate” in order to defend American national security and enforce U.N. resolutions regarding Iraq — left it up to the president to decide whether and when to initiate the war.

After voting for the resolution, prominent members of Congress insisted they hadn’t voted to use force. That was for the president to decide. Former senator Tom Daschle said: “Regardless of how one may have voted on the resolution last night, I think there is an overwhelming consensus … that while [war] may be necessary, we’re not there yet.” And in 2007, Hillary Clinton’s campaign chairman Terry McAuliffe explained that Senator Clinton never voted for the Iraq war: instead, she voted “to give the president the authority to negotiate and to have a stick to go over there and negotiate with Saddam Hussein.”

Biden’s post-hoc rationalizations for his vote follow the same pattern. “If I had known this administration would be so incompetent,” he told interviewer Charlie Rose in 2004, “I never would have given them the authority to try to avoid the war,” which is an interesting way to describe a resolution titled “Authorization for Use of Military Force Against Iraq.” In fact, Biden is at least as complicit in the decision to go to war as is Republican presidential candidate John McCain. Biden joined with McCain to defeat an amendment offered by Senator Carl Levin that would have forced President Bush to get U.N. Security Council approval before launching the invasion.

It’s still hard to get a firm answer from Biden as to whether he supported the invasion and occupation of a country that represented no serious threat to the United States. But he insists that he was firmly against botching the job. Well…who wasn’t?

Such evasiveness is all too common in Congress today. The power of the presidency continues to grow largely because many legislators want to duck their responsibility to decide the question of war and peace, delegate that responsibility to the president, and reserve their right to criticize him should military action go badly — to be for it before they were against it, or vice versa, depending on which way the political winds blow.

Congressional scholar Louis Fisher compares the Iraq vote to the Gulf of Tonkin resolution that empowered Lyndon Johnson to expand the Vietnam War. As with the Iraq-war resolution, the Gulf of Tonkin resolution was worded broadly enough to allow the president to make the final decision about war all by himself. Lyndon Johnson compared the resolution to “grandma’s nightshirt” because it “covered everything.” And, as with Iraq, the president waited for several months and an intervening election before using the authority granted him. In each case, it was easier for Congress to dodge the issue than to take responsibility.

That is not how our Constitution is supposed to operate. As Madison put it: “Those who are to conduct a war cannot in the nature of things, be proper or safe judges, whether a war ought to be commenced, continued, or concluded. They are barred from the latter functions by a great principle in free government, analogous to that which separates the sword from the purse, or the power of executing from the power of enacting laws.”

That’s a principle that gets very little respect from today’s accountability-shy legislators. Shortly before the vote, Biden declared, “I am fully confident if the president decides, in concert with others, war is necessary, he will have to inform [the American people] before he launches it.” But it is not for the president to decide whether war is necessary. The Constitution leaves that decision to Congress. That prominent senators — and vice-presidential candidates — squirm to avoid responsibility for it, does not bode well for the future health of either branch.