WASHINGTON -- The U.S. Senate is headed for a vote on legislation outlawing workplace discrimination against gay, bisexual and transgender Americans. Senate passage would be a major victory for gay rights advocates, but the bill appears to have little chance of passing the Republican-controlled House of Representatives.

The bill's passage through the Senate alone would demonstrate the country's quickly evolving attitude toward gay rights, even if it doesn't become law. The Supreme Court in June affirmed gay marriage and granted federal benefits to legally married same-sex couples. Illinois is on the verge of becoming the 15th state to legalize gay marriage.

Federal law prohibits discrimination on the basis of sex, race and national origin. But it doesn't stop an employer from firing or refusing to hire workers because they are gay, lesbian, bisexual or transgender.

While the Senate bill would change that, the Republican-led House is unlikely to even vote on it.

If the House fails to act on the bill, gay rights advocates are likely to press President Barack Obama to act unilaterally and issue an executive order barring anti-gay workplace discrimination by federal contractors.

Republican House Speaker John Boehner is maintaining his longstanding opposition to the measure, arguing that it is unnecessary and certain to create costly, frivolous lawsuits for businesses. Outside conservative groups have cast the bill as anti-family.

That didn't stop proponents from stepping up the pressure on the eve of the vote.

All 55 members of the Democratic majority in the Senate and several Republicans were expected to unite on Thursday in backing the Employment Non-Discrimination Act.

"I hope that we are on the verge of making history tomorrow by passing this bill with a strong vote," Republican Sen. Susan Collins, said Wednesday. "I then hope that our colleagues on the House side will follow suit and that we can see this bill signed into law."

Sen. John McCain, the Republican presidential nominee in 2008, signalled his conditional support on Wednesday.

Not a single opponent stood on the Senate floor to speak out against the bill, a remarkable silence.

Likely Senate approval of the overall bill reflects America's growing tolerance of gays and Republican political calculation as it looks for supporters beyond its core base of older voters.

A Pew Research survey in June found that more Americans said homosexuality should be accepted rather than discouraged by society by a margin of 60 per cent to 31 per cent. Opinions were more evenly divided 10 years ago.

Republican-leaning groups such as the American Unity Fund, which counts on hedge fund billionaires as well as former Republican lawmakers, pushed for the legislation.

Through three days of Senate debate, backers of the bill repeatedly described it as an issue of fairness some 50 years after Congress passed and President Lyndon Johnson signed the Civil Rights Act, the landmark legislation that banned racial segregation and other kinds of discrimination.

The bill would bar employers with 15 or more workers from using a person's sexual orientation or gender identity as the basis for making employment decisions, including hiring, firing, compensation or promotion. It would exempt religious institutions and the military.

By voice vote Wednesday, the Senate approved an amendment from that would prevent federal, state and local governments from retaliating against religious groups that are exempt from the law.

The Senate planned to vote Thursday on an amendment to expand the number of groups that are covered under the religious exemption.

Twenty-two states and the Washington capital district have approved laws banning workplace discrimination on the basis of sexual orientation, and 17 of those also prohibit employers from discriminating based on gender identity.

About 88 per cent of Fortune 500 companies have adopted nondiscrimination policies that include sexual orientation, according to the Human Rights Campaign.