The legislation, known as a war powers resolution, failed on a 47-53 vote tally. The vote fell mostly along party lines, though Republican Senator Rand Paul of Kentucky voted in favour and Democratic Senator John Fetterman of Pennsylvania voted against.
The war powers resolution gave lawmakers an opportunity to demand congressional approval before any further attacks are carried out.
The vote on Wednesday, US time, forced them to take a stand on a war shaping the fate of US military members, countless other lives and the future of the region.
Underscoring the gravity of the moment, Democratic senators filled the Senate chamber and sat at their desks as the voting got under way. Typically, senators step into the chamber to cast their vote, then leave.
"Today every senator - every single one - will pick a side," Senate Democratic Leader Chuck Schumer said before the vote.
"Do you stand with the American people who are exhausted with forever wars in the Middle East or stand with Donald Trump and Pete Hegseth as they bumble us headfirst into another war?"
Senator John Barrasso, second in Senate Republican leadership, said during the debate that Republican senators would send a message that Democrats are wrong for forcing a vote on the war powers resolution.
"Democrats would rather obstruct Donald Trump than obliterate Iran's national nuclear program," he added.
Defence Secretary Pete Hegseth said on Wednesday the war could extend eight weeks, a longer time frame than has previously been floated by the Trump administration.
He also acknowledged that Iran is still able to carry out missile attacks even as the US tries to control the nation's airspace.
US service members "remain in harm's way, and we must be clear-eyed that the risk is still high", General Dan Caine, chairman of the Joint Chiefs of Staff, said at the same press conference.
Six US military members were killed over the weekend in a drone strike in Kuwait.
Republican Senator Joni Ernst of Iowa acknowledged the human costs of the war in her floor speech.
One of the soldiers killed on Sunday was from Iowa and a National Guard unit from her state was also attacked in Syria in December, resulting in the deaths of two other soldiers.
"But now is our opportunity to bring an end to the decades of chaos," said Ernst, who herself served as an officer in the Iowa National Guard for two decades.
"The sooner the better," she added.
