Kirk delivered his first speech on the Senate floor since suffering a stroke in January 2012. Seated at a desk, Kirk said it was especially important for an Illinois Republican to speak out for the legislation in the tradition of Everett Dirksen and Abraham Lincoln, two leaders on civil rights.
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. No senator spoke in opposition to the measure during Monday's debate.
Tony Perkins of the conservative Family Research Council said in a statement that he was disappointed in the Senate vote, but "confident that the U.S. House of Representatives will ultimately reject ENDA because it not only threatens the free market but religious liberties as well."
Current 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.
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.
The bill would exempt religious institutions and the military.
Opening Senate debate, Majority Leader Harry Reid, D-Nev., quoted slain gay rights leader Harvey Milk, who argued that freedom and individual rights shouldn't hinge on political deals and opinion polls.
The law, Reid said, would ensure that "all Americans regardless of where they live can go to work unafraid to be who they are." Sen. Tom Harkin, D-Iowa., called the measure another step forward in the country's progress.
Meanwhile, in Maine, Michaud wrote about his homosexuality.
"That may seem like a big announcement to some people. For me, it's just a part of who I am, as much as being a third-generation mill worker or a lifelong Mainer. One thing I do know is that it has nothing to do with my ability to lead the state of Maine," Michaud wrote in an op-ed article.
The anti-discrimination bill faces strong opposition from conservative groups — Heritage Action and the Faith and Freedom Coalition said the vote will be part of their legislative scorecard on lawmakers. More to its immediate prospects, the legislation is opposed by Boehner, casting doubt on whether the House will vote.
Reiterating Boehner's longstanding opposition, spokesman Michael Steel said Monday that Boehner "believes this legislation will increase frivolous litigation and cost American jobs, especially small business jobs."
Chad Griffin, president of the Human Rights Campaign, the largest gay and lesbian advocacy group, contrasted Heller's backing with Boehner's opposition.
"The speaker, of all people, should certainly know what it's like to go to work every day afraid of being fired," Griffin said, a reference to the unsuccessful, tea party-backed challenge to Boehner earlier this year.
Twenty-two states and the District of Columbia 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 percent of Fortune 500 companies have adopted nondiscrimination policies that include sexual orientation, according to the Human Rights Campaign. About 57 percent of those companies include gender identity.
Associated Press writers Josh Lederman, Nedra Pickler and Alan Fram contributed to this report.