The Senate on Saturday approved repealing the "don't ask, don't tell" policy that bans homosexuals from serving openly in the military.
The repeal passed 65-31 despite letters and calls from conservative organizations concerned with the effect of a repeal during a time of war.
"Today is a tragic day for our armed forces," said Family Research Council President Tony Perkins. "The American military exists for only one purpose – to fight and win wars. Yet it has now been hijacked and turned into a tool for imposing on the country a radical social agenda.
"This may advance the cause of reshaping social attitudes regarding human sexuality, but it will only do harm to the military's ability to fulfill its mission."
Days earlier, the House passed an identical bill against the 1993 policy.
President Barack Obama is expected to sign the legislation next week.
"It is time to close this chapter in our history," Obama said in a statement. "It is time to recognize that sacrifice, valor and integrity are no more defined by sexual orientation than they are by race or gender, religion or creed."
Perkins accused Congress of catering to a small base of the Democratic Party and trying to ram through a repeal in the lame-duck session.
"It is clear why this was done: not to enhance the military's ability to accomplish its mission or to enhance national security," he said. "Rather, it is a political payoff to a tiny, but loud and wealthy, part of the Democratic base. They knew that the Congress elected last month would never adopt such legislation – certainly not without a more thoughtful and deliberative process."
"Don't ask, don't tell" was enacted by President Clinton in 1993 after Congress passed a law that same year banning homosexuals from serving in the military. Though it bars openly gay individuals from serving in the U.S. military, it also bars the military from asking service members their sexual orientation.
The Senate previously rejected a repeal effort twice this year.
The bill will not take immediate effect. Obama and his top military advisers, including Defense Secretary Robert Gates, must certify that lifting the ban will not adversely affect the military. That is then followed by a 60-day waiting period as new rules are written for the military.