While President Obama dithers, Congress needs to act against ISIS. House and Senate leaders should reconvene Congress this week and take the unprecedented step of authorizing military action against ISIS for President Obama’s signature. Customarily, a president takes the first step, presenting a draft Authorization for the Use of Military Force (similar to a declaration of war), then negotiating with Congress on the goals and scope. But Obama’s intentions are unknown, so Congress should seize the initiative.

The world is waiting for an American response to the beheading of James Foley and ISIS’s explicit threats to attack this country and raise its flag over the White House. Delay and ambivalence – hallmarks of Obama’s foreign policy – will discourage allies and embolden the enemy. Congress can step in, while sparing a reluctant president from having to expend political capital on war. After all, it is to Congress that the Constitution grants the power — and responsibility — to declare war.

Since the tenure of our second president, John Adams, Congress has given the Commander in Chief permission to use military force on twelve significant occasions without actually declaring war. Early on, Adams, Thomas Jefferson, and James Madison requested permission to fend off pirates and unfriendly nations attacking American shipping. Since World War II, congressional authorizations have virtually replaced declarations of war, including in Vietnam in 1964, Iraq in 1991 and 2002, and the “war on terror” following the attack on 9/11/2001.

Recent presidents, including both Bushes, insisted the authorizations they requested were not constitutionally required. George W. Bush said he was seeking support, not permission, to invade Iraq in 2002 to show the world that “the United States speaks with one voice.” But, in truth, Mr. Bush settled for a narrower authorization to fight in Iraq, after getting turned down for permission to “restore international peace and security in the region.”

As Mr. Bush learned, not all authorizations are equal. In meeting the current threat, Congress will have to decide whether the goal is containment of ISIS or its destruction. Both the enemy and foreign allies will be watching to see if Congress limits the territorial scope of battle, duration, or the type of forces employed. The chairman of the joint chiefs, General Dempsey, said last week that ISIS could be contained, but “not in perpetuity.” Noting its “apocalyptic” vision, the general said that it “will eventually have to be defeated” and that will require U.S. forces going into Syria.

There are, on the one hand, unlikely voices calling for ISIS’s destruction, including a former chairman of the Democratic National Committee, Howard Dean. Senator Nelson of Florida, a senior Democrat on the Senate Armed Services Committee, advocates targeted air strikes in Syria. On the other hand, the House Democratic Whip, Steny Hoyer, says that while he supports the current air strikes, the President must come to Congress if he wants to broaden the campaign. Several Democratic lawmakers are voicing this concern about “mission creep.” There are likely enough Democratic votes to support authorization, and the number will grow as the atrocities continue.

The burden of leadership has fallen on Congress, with Mr. Obama so far showing no sign of asking for authorization. In response to the beheading of an American citizen, he vowed that “when people harm Americans anywhere, we do what’s necessary to see that justice is done.”

What does that mean? Mr. Obama has often said that he prefers to fight terrorists in court. At the National Defense University in May 2013, Mr. Obama said America should not use military force “when we have the ability to capture individual terrorists; our preference is always to detain, interrogate, and prosecute.” The Justice Department already announced it’s pursuing a criminal investigation against Foley’s killers.

That’s illogical. It’s also not the strategy recommended by the president’s own Joint Chiefs, considering the hundreds of terrorists with western passports vowing our destruction and the training camps in Syria and Iraq spawning more.

Sadly, it looks like politics could take precedence over the nation’s safety. Senate Majority Leader Harry Reid is reportedly skittish about a vote on military action before the mid-term elections. It could anger his party’s base and endanger Democratic incumbents. Members are safest, said Representative Brad Sherman, a Democrat of California,“if they limit their activities to riding in parades.”

Governor McCaughey, former lieutenant governor of New York, is author of “Government by Choice: Inventing the United States Constitution.”