WASHINGTON (Reuters) – U.S. lawmakers will grill top Trump administration officials on Monday about a new authorization for the use of military force for the campaign against Islamic State and other militant groups, Congress’ most significant step in years toward taking back control of its constitutional right to authorize war.

President Donald Trump’s secretary of state, Rex Tillerson, and Secretary of Defense Jim Mattis will testify before the Senate Foreign Relations Committee at a hearing on the administration’s view of a new Authorization for the Use of Military Force, known by the acronym AUMF.

Republican and Democratic members of Congress have been arguing for years that Congress has ceded too much authority over the deployment of U.S. forces to the White House in the wake of the Sept. 11, 2001, attacks. But they are also divided over how much control they should exert over the Pentagon. Efforts to write a new AUMF have failed for years.

Under the Constitution, Congress, not the president, has the right to declare war.

Concerns intensified this month after four U.S. soldiers were killed in Niger. Several complained that the Pentagon had not been providing enough information about the ambush.

“What’s happening in Niger and more broadly in Africa suggests a greater urgency for an AUMF,” Democratic Senator Tim Kaine, a leading advocate for a new authorization, told reporters on Thursday after a classified briefing on the Niger operation by Pentagon officials.

“I think the extent of the operation, the number of countries, will be surprising to people,” he said, adding that he would raise it at the hearing.

Republican Senator John McCain, chairman of the Senate Armed Services Committee and the chamber’s most famous war veteran, said last week he may consider issuing a subpoena because the White House had not been forthcoming with details of the Niger attack and threatened to block Trump nominees.

Congress has not passed an AUMF since the 2002 measure authorizing the Iraq War. But the legal justification for most military action for the past 15 years is an older AUMF, for the campaign against al Qaeda and affiliates after the Sept. 11, 2001, attacks.

Backers of a new AUMF say the 2001 authorization, which was not limited by time or geography, has let presidents wage war wherever they like, without spelling out any strategy for Congress, or the public. For example, Islamic State did not exist when the 2001 AUMF was passed.

Trump’s fellow Republicans control majorities in both the Senate and House of Representatives, but there are deep divisions over any AUMF within the party, as well as between Republicans and Democrats.

Many Republicans, like McCain and Senator Lindsey Graham, do not want an AUMF that exerts too much control over the Pentagon. They argue that military commanders should decide how best to fight America’s enemies.

Many Democrats say they want an AUMF that imposes limits on why, where, and for how long U.S. forces can be sent to fight.