The US Senate is to vote on a bill which contains a measure to repeal the ban on out gay soldiers in the military.
The measure has been added to the National Defence Authorisation Act and Democrat senators need 60 votes for it to progress to a debate on the floor.
The vote will take place on Tuesday evening.
The annual bill usually passes the vote but the controversial issue of out gay soldiers means it is not certain to succeed.
Republican senator John McCain has promised to filibuster the bill. This is an obstruction strategy used to delay a vote.
A review of the policy, known as Don't Ask, Don't Tell, is underway and is expected to be completed by the end of the year.
Mr McCain has said that the Senate should wait until the review is completed and that lifting the ban will lower morale and damage the military.
Yesterday, bisexual singer Lady Gaga joined a march in Portland, Maine, to call for repeal of the policy and urge Republican Senators Olympia Snowe and Susan Collins to support the repeal effort.