Senators Kaine and Corker were backed by a group of four other lawmakers when they introduced their AUMF bill on Monday. The bill would require presidents of the United States to alert relevant committees in the US Congress before adding a group to the list of entities the United States can use military force against, although the bill explicitly makes exemptions for "sovereign nations."
In the past, the president could only authorize force against groups associated with al-Qaeda or the Taliban or others that had a hand in the 9/11 terrorist attacks; the 2012 National Defense Authorization Act (NDAA) widened the scope to permit force against groups who supported them as well. All this could be done secretly, without telling Congress, as permitted by the 2001 AUMF.
Former US President Barack Obama used the 2001 AUMF to go to war with Daesh, although the group had a high-profile split from al-Qaeda and therefore was not an affiliate, with his administration arguing it was a "legally insignificant fact."
The new bill expands the list of terrorist groups the United States can strike, allowing new additions so long as the president updates the list and tells Congress. Under the bill, the president's war powers would also have to be reauthorized by Congress every four years.
The question of whether this AUMF would allow the president to indefinitely detain an American without trial remains as unanswered as ever. Nothing in the new bill specifically amends the language pertaining to US citizens, however.
Former President George W. Bush used the 2001 AUMF to put José Padilla, an American citizen, in military detention, and the US Supreme Court never ruled on whether that was legal. The Trump administration might decide do so again, in the case of an unnamed American caught fighting with Daesh.
The issue has gone back and forth since then. A three-judge panel from the Court of Appeals for the US 4th Circuit concluded in 2005 that Padilla's detention was legal.
Obama appeared to believe that he could also detain Americans similarly, but chose not to, saying, "my administration will not authorize the indefinite military detention without trial of American citizens," when signing the 2012 NDAA. He said the bill "does nothing more than confirm authorities that the federal courts have recognized as lawful under the 2001 AUMF."
Then, in May 2012, a judge ruled against the constitutionality of indefinite detentions like Padilla's in a lawsuit brought by journalists fearful that the law could affect their sourcing.
In July 2013, the 2nd Circuit Court of Appeals vacated the injunction in the case of the journalists, arguing that the NDAA did not apply to the government's authority to detain American citizens indefinitely and without trial.
Thus Trump could, theoretically, add an American or an American entity to the list. However, such a move would probably receive extreme blowback from Congress. Meanwhile, the new AUMF bill does not hold high prospects for passing the Republican-controlled House of Representatives and Senate. Kaine has been advocating for reform of the AUMF for years, even under fellow Democrat Obama. Meanwhile, Corker has announced that he plans to retire at the end of his term in 2018.
"The administration is not seeking a new AUMF, as the US has sufficient legal authority to prosecute the campaign against the Taliban, al-Qaeda and associated forces, including against ISIS [Daesh]," a National Security Council official reportedly said back September. The White House has remained mum on the issue since the introduction of Corker and Kaine's bill.