The Senate voted 61-36 in a procedural motion to kill Paul's amendment, which would have repealed the two Authorizations for Use of Military Force after six months, giving Congress a window to draft a new, updated war authorization.
Paul, a Kentucky Republican, has argued that the 2001 and 2002 war authorizations -- which were passed to authorize military action against Al Qaeda and in Iraq -- are hopelessly out of date as the military focuses on fighting ISIS across the Middle East, a group that didn't exist at that time.
While Paul's amendment to the National Defense Authorization Act (NDAA) was unsuccessful, he said it was a victory just to get the Senate to debate and take a vote on the war authorizations.
"It's the first time in 15 years to actually debate what our role is in declaring an initiation of war," Paul told reporters just before the vote. "The problem with waiting for the AUMF first, for a new authorization, is we've been waiting 16 years, and it's not coming without somebody giving somebody a kick in the seat in the pants."
The vote did not break down along party lines. Democratic Sens. Elizabeth Warren of Massachusetts and Tammy Duckworth of Illinois were among the Democrats joining Paul in his effort to sunset the current war authorizations.
All of the Democrats generating buzz about a potential 2020 presidential run, including Warren, Sens. Cory Booker of New Jersey, Kamala Harris of California and Chris Murphy of Connecticut, voted with Paul.
Thirteen Democrats voted with most Republicans to defeat the measure
Just two Republicans other than Paul, Sens. Mike Lee of Utah and Dean Heller of Nevada, voted for the amendment.
Paul was also opposed by both Republicans and Democrats, including the two leaders of the Senate armed services committee, Republican John McCain of Arizona and Democrat Jack Reed of Rhode Island.
"To rescind the AUMF that authorizes action against these forces -- and to leave nothing but uncertainty for our deployed forces and for our allies -- is simply irresponsible, and it breaks faith with our volunteer force," Senate Majority Leader Mitch McConnell said.
The Trump administration -- and the Obama administration before it -- argued they have sufficient legal authority to carry out US military operations against ISIS and Al Qaeda, and that a new one is not needed, although the Trump administration has indicated it isn't outright opposed to the effort to pass a new one.
Wednesday's vote was the second time this year that Congress has taken action targeting the 2001 war authorization, which was passed just days after the Sept. 11, 2001, terrorist attacks.
In June, the House appropriations committee approved an amendment
from California Democratic Rep. Barbara Lee to sunset the 2001 war authorization. Lee was the only member in the House to vote against the authorization when it passed in 2001.
But congressional leaders have been weary of passing a new war authorization, and House Republicans stripped Lee's amendment
out of the defense appropriations bill when it was passed on the floor.
In the Senate, Paul forced a vote on his amendment by threatening to object to all other amendments on the NDAA, the massive defense policy bill the Senate is considering this week.
Had Paul done so, it would have effectively killed any other amendment votes.
Paul's effort isn't the only one ongoing in the Senate to pass a new war authorization. Virginia Democratic Sen. Tim Kaine and Arizona Republican Sen. Jeff Flake have introduced a war authorization
that the Senate foreign relations committee may consider, which would also sunset the 2001 and 2002 war authorizations with a new five-year authorization for US military operations against ISIS, al Qaeda, the Taliban and "associated" forces.
But Kaine and Flake split on Paul's amendment.
Kaine said he supported Paul's effort, arguing that it was "way past time for Congress to take this up and for everybody to be on the record."
Flake, however, argued that he supported Paul's goal, but not his method of repealing the current war authorization without a new one in place.
"I cannot support my colleague's effort to repeal the 2001 AUMF in 6 months because of the very real risk associated with repealing such a vital law before we have something to replace it with," Flake said.
Senate foreign relations chairman Bob Corker said he agreed with Paul that Congress needs to take action on a new war authorization, but said he was opposed to the substance of Paul's effort.
"Doing away with the legal basis under which we're going against ISIS today, before we've implemented and put in place another one, to me is not prudent," Corker said.