Business groups such as the U.S. Chamber of Commerce have remained neutral on the issue.
Sens. Rob Portman, R-Ohio, and Pat Toomey, R-Pa., were crafting amendments to the bill dealing with religious exemptions.
Seven Republicans and 54 Democrats stood together Monday and cleared the bill past its first hurdle on a 61-30 vote, setting the stage for possible passage by week's end. The Employment Non-Discrimination Act would prohibit workplace discrimination against gay, bisexual and transgender Americans.
The legislation, the first significant gay rights bill since Congress ended the ban on gays serving openly in the military in 2010, faces strong opposition in the House, with Boehner rejecting the measure.
Final passage would cap a 17-year quest to secure Senate support for a similar discrimination measure that failed by one vote in 1996, the same year Congress passed and President Bill Clinton signed the Defense of Marriage Act, the law prohibiting federal recognition of same-sex marriage.
"I think back to Martin Luther King's commentary that the great arc of the universe bends toward justice and I feel that our notion of fairness about employment, how central that is to pursuit of happiness, how central it is to equality, how central it is to the golden rule ... means that we will accomplish this," said Sen. Jeff Merkley, D-Ore., a chief sponsor of the bill. "But I do hope it's sooner rather than later."
Americans have displayed a greater acceptance of homosexuality while Republicans, who struggled to win over young people and independents in the 2012 presidential election, have searched for supporters beyond their core base of older voters. The Supreme Court in June affirmed gay marriage and granted federal benefits to legally married same-sex couples while same-sex marriage is legal in 14 states and the District of Columbia.
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 percent to 31 percent. Opinions were more evenly divided 10 years ago.
About a half hour after the Senate acted, President Barack Obama cited the vote as an example of "common sense starting to prevail" in a Congress that has opposed much of his agenda.
"Inexorably, the idea of a more tolerant, more prosperous country that offers more opportunity to more people, that's an idea that the vast majority of Americans believe in," the president told a group of supporters gathered for a summit in Washington Monday night.
In Maine, six-term Democratic Rep. Mike Michaud, who is running for governor, said he was gay and questioned whether it still mattered to voters, a stark reminder of changing views, lingering resistance to homosexuality and uncertainty about the political implications.
"Attitudes are changing very rapidly on gay rights issues and we're seeing that with each passing day. More and more people have embraced equality," said Sen. Susan Collins, R-Maine, a sponsor of the measure.
The three potential Republican presidential candidates — Marco Rubio of Florida, Ted Cruz of Texas and Rand Paul of Kentucky — voted against, a reflection that among core GOP conservative voters opposition to gay rights remains strong.