Although the Constitution gives Congress the power to declare war, the United States military kept a large standing force deployed around the world as World War II gave way to the Cold War. Presidents, invoking their role as commander in chief, had directed those forces to launch or escalate wars, including in Korea and Vietnam.

One part says presidents may only introduce forces into hostilities after Congress has authorized using force or if the nation has been attacked. No subsequent president has respected that narrow list of when he may unilaterally dispatch forces into combat.

Another part requires presidents to consult with Congress before deploying troops into actual or imminent “hostilities.” Most presidents have obeyed this, but Mr. Trump did not before ordering the Suleimani strike.

Yet another part — important here — says if a president deploys combat troops without authorization, the deployment must end after 60 days unless lawmakers approve it in the interim. It also empowers Congress to direct the president to terminate the operation before that deadline.

What are Democrats trying to do?

They are trying to use the War Powers Resolution to block a war with Iran.

Senator Tim Kaine, Democrat of Virginia, has already proposed a joint resolution to do so, and Speaker Nancy Pelosi said on Sunday that the House would act this week on a similar measure. The House version’s sponsor will be Representative Elissa Slotkin, Democrat of Michigan and a former C.I.A. and Pentagon analyst specializing in Shiite militias.