The Republicans in the Senate seem to have had no trouble inserting themselves into the Iran nuclear negotiations, when they had no business interfering. Yet they have shown little interest in carrying out a job that is squarely within their constitutional mandate — drafting an authorization for war against ISIS that Democrats can support and President Obama will sign.

The Republicans are the leaders in Congress, but their efforts to undermine Mr. Obama in every matter are infecting all governance. Unless something shifts soon, Congress may not be able to come up with any authorization and will fail to put necessary limits on American involvement so that the country does not become embroiled in another prolonged ground conflict as it did in Iraq and Afghanistan.

Mr. Obama insists he has ample legal authority to continue the war against the Islamic State, called ISIS or ISIL, under previous authorizations for the use of military force (one passed by Congress in 2001 to launch the fight against Al Qaeda, the other in 2002 to invade Iraq). But those authorizations never contemplated a separate military conflict against ISIS, which is already in its eighth month.

Mr. Obama sent a proposed new authorization to Congress in February that has a three-year time limit and restrictions on the use of ground forces. Democrats are correct in their analysis that the measure is not restrictive enough; Republicans want fewer constraints on military force, even though many doubt the effectiveness of the administration’s anti-ISIS strategy.