*** UPDATE *** 4:45 PM ET: In a press conference after the vote, Sens. Joe Lieberman and Susan Collins announced that they will introduce a stand-alone bill to repeal "Don't Ask Don't Tell" and that Senate Majority Leader Reid has promised to support it. Reid told Lieberman that he will bring the legislation up for a vote before the end of the lame duck session, the Connecticut lawmaker said.
"We're not going to give up," Lieberman said. "We're going to keep fighting until the last possible minute in the session."
Collins said that she was "very disappointed" that Reid "walked away from negotiations" and brought the bill up for a vote before an agreement on the process of the debate had been reached and the 60 votes needed to move forward were assured.
If the Senate approves the standalone measure, the House would have to approve it as well.
4:08 pm ET: A key procedural vote on the bill containing a repeal of the military's Don't Ask Don't Tell policy failed Thursday, likely dealing a final blow to advocates who hoped to overturn the 17-year old ban on gays and lesbians serving openly in the military during this session of Congress.
Democrats needed 60 votes to advance the Defense Authorization bill for debate on the floor. The vote failed, 57-40.
Ultimately, Majority Leader Harry Reid called for the vote without having reached a procedural agreement with moderate Republican Sen. Susan Collins of Maine, who supports repeal but wanted greater openness for the process of amending and passing the bill. Collins voted aye on the measure, but other Republicans who support repeal but had voiced similar procedural concerns -- Sens. Scott Brown and Lisa Murkowski -- voted no.
One Democrat, newly-elected Sen. Joe Manchin of West Virginia, broke with his party to vote no.
The result means that repeal of the ban, enacted in 1993, is unlikely to be changed by Congress anytime soon. The policy is also currently being considered in court proceedings.
Supporters of repeal, including Adm. Mike Mullen and Defense Secretary Robert Gates, have warned that a failure of congressional action could mean a hasty and disorderly implementation of a change if the courts overturn the policy first.
Opponents of repeal say that a change in policy during wartime could disrupt the effectiveness and safety of troops in combat. A recent survey showed that troops serving in combat and members of the Marine Corps were more likely than other servicemembers to voice concerns about the consequences of repeal.
In remarks shortly before the vote, Reid blamed Republicans – but not Collins – for blocking the massive defense measure from coming to the floor.
"It's quite clear that they're trying to run out the clock," Reid said of GOP opponents before calling for the vote.
Reid went out of his way to praise Collins for trying to reach an arrangement that would have paved the way for Democrats to win the 60 votes they need to advance the measure containing the DADT language. "She's tried," Reid said.
Collins said on the Senate floor that she was "perplexed and frustrated" that the bill would fall victim to "politics."
"I just do not understand why we can't proceed on a path... that will allow us to get the 60 votes to proceed," she said.
NBC's Ken Strickland contributed