AMONG THE business that Congress will leave unfinished this month is legal authorization of the war against the Islamic State. Though the war has been underway for five months, President Obama has said he would welcome legislation, and congressional leaders have denounced the president’s unilateral actions in other spheres, neither the White House nor Congress has made a passage of an Authorization for Use of Military Force a priority. That puts the ongoing military operations on shaky legal ground and deprives them of the political mandate they ought to have.

Democrats on the Senate Foreign Relations Committee did succeed in forcing a committee vote last week on an authorization that would have sanctioned three years of military operations against the Islamic State while repealing the 2002 authorization of force for Iraq. Both the Obama administration and Republicans objected to limits the legislation placed on the use of ground forces. While trainers and some special forces missions were allowed, the more general use of ground forces was prohibited.

Sen. Tim Kaine (D-Va.), one of the few in Congress to push hard for an authorizing vote, argues that if U.S. troops were needed to head off an attack on the United States — the circumstance Mr. Obama has cited that would alter his own opposition to combat forces — the president could act under his constitutional authority. That dodges the more simple and sensible conclusion that Congress’s role is to authorize wars and their aims, not micromanage how they are waged. As Secretary of State John F. Kerry told the Foreign Relations Committee, it would be a mistake to “preemptively bind the hands of the commander in chief or our commanders in the field in responding to scenarios and contingencies that are impossible to foresee.”

Mr. Kaine and the outgoing Foreign Relations chair, Sen. Robert Menendez (D-N.J.), at least made a serious effort to forge a bill and get it passed. That’s more than can be said for the White House, which despite saying that it wanted congressional authorization declined to submit its own legislation. It’s the first time a president has asked for war authority without providing Congress with a draft. Mr. Obama contends that the military operations in Iraq and Syria are justified by the 2002 authorization on Iraq and a 2001 authorization aimed at al-Qaeda; but that stretches those statutes beyond what they were intended to address.

The president’s passivity has been matched by congressional Republicans, who have cited Mr. Obama’s failure to submit legislation as a reason not to move on an authorization. The Foreign Relations Committee’s incoming chair, Sen. Bob Corker (R-Tenn.), says he wants to see the administration spell out a coherent plan for Syria. While he’s correct in identifying the biggest gap in Mr. Obama’s war strategy, that shouldn’t be a reason to delay a vote on the war.

Putting off an authorization makes both the Obama administration and Congress less accountable for what is likely to be a long and difficult conflict. Rather than continue to dodge their legal and political duty, the White House and congressional leaders should make passage of a war authorization one of the first acts of the next Congress.