WASHINGTON — Senator Joe Manchin III, Democrat of West Virginia, said Wednesday that he would vote for a nondiscrimination bill that protects gay men, lesbians and transgender people, putting the measure within one vote of gaining the support it needs to overcome a filibuster.

Mr. Manchin was the only Democrat who had not signaled how he would vote. His support means that all 55 members of the Democratic majority are expected to get behind the bill, known as the Employment Nondiscrimination Act, which has languished in various forms in Congress for nearly 40 years. Senator Harry Reid of Nevada, the majority leader, could bring it to the floor as early as next week.

“There’s no way that I could ever not support something that basically bans discrimination,” Mr. Manchin said. “There’s no way. It’s just a fundamental right.”

Four Republicans have also expressed support, meaning there are now 59 senators who are on record as favoring the measure. Sixty votes are needed to overcome a filibuster.