Such votes are rare in part because, for the past two decades, presidents have relied on the 2001 Authorization for Use of Military Force (AUMF), which Congress passed after the Sept. 11, 2001, terrorist attacks. But nearly 20 years later, presidents increasingly must resort to contorted interpretations of the law to argue that it covers new conflicts. That may be leading lawmakers to turn to the War Powers Act, because they are reluctant to rewrite the AUMF.