Monday night, the Senate took a key vote on the Employment Non-Discrimination Act, virtually assuring its passage in the upper chamber. It's the biggest step forward yet for an effort that's been 30 years in the making, but still faces long odds as it heads to the House, since Speaker John Boehner says he won't bring it to a vote. Here's what you need to know.

What is ENDA?

ENDA is the acronym for the Employment Non-Discrimination Act, which would prohibit discrimination on the basis of sexual orientation or identity in hiring decisions for businesses with more than 15 employees.

Is that kind of discrimination really a problem?

As summarized in a 2012 paper by the Williams Institute at the University of California Los Angeles School of Law, harassment of gay, lesbian, bisexual and transgender employees continues to be pervasive, even as more companies have adopted non-discrimination policies. Studies over the past decade have found that GLBT people tend to be paid less than their straight colleagues, have less access to health insurance, more often feel that they were fired or denied promotions, and often hear disparaging comments based on their sexual orientation, even if they're not out at work.

In 2007, the American Civil Liberties Union rounded up a bunch of real-life examples of what workplace discrimination looks like.

So why hasn't anyone done anything about it?

At the moment, 22 states have their own statutes that prohibit discrimination in hiring on the basis of sexual orientation, 18 of which include sexual identity. A Government Accountability Office report from July found that "relatively few" complaints were filed in states with protections, which include the biggest states where most GLBT people reside. That's lessened urgency for a federal law, though advocates say the patchwork of local protections is inadequate.

What would ENDA do?

Title VII of the Civil Rights Act already prohibits employment discrimination on the basis of race, religion, gender, national origin, age and disability. The bill would extend that protection to GLBT people, making it unlawful for companies, labor unions, and employment agencies to hire, fire, promote, or compensate people differently based on sexual orientation or sexual identity. In order to make it palatable, though, it's got some big exceptions: The protections would not apply to religious organizations, members of the armed forces, or companies with fewer than 15 employees.

Why's it just passing now?

ENDA has been in the works in some form since 1974, when Rep. Bella Abzug (D-N.Y.) introduced the "Equality Act," which would have protected gays and lesbians from discrimination in the workplace. The current version, minus protections for transgender people, was introduced in 1994, and has been in almost every Congress since then. The gay-rights movement has racked up significant victories in recent years, including the repeal of "Don't Ask Don't Tell," and many lawmakers publicly announced their support for same-sex unions around two landmark Supreme Court decisions earlier this year. Prohibiting employment discrimination is one of the last big battles left.

Who's opposing it, and why?

The fiercest opposition comes from conservative Christian groups like the American Family Association, which says the bill infringes on the First Amendment, and the Family Research Council, which argues the bill would "approvingly bring private behavior considered immoral by many into the public square." The more mainstream U.S. Conference of Catholic Bishops also thinks ENDA contains inadequate protections for religious expression -- forbidding "as discrimination what is legitimate, moral disapproval of same-sex conduct" -- and could shore up arguments (already well underway) for the legalization of gay marriage.

Here's what the Family Research Council fears will happen in the United States if it goes the way of Canada:

For his part, Boehner says that ENDA's aims are already accomplished by current law, but hasn't specified what law he means (an inquiry to the Speaker's office hasn't yet been returned). He also worries that including sexual orientation and gender identity as a protected class would lead to more "frivolous litigation" and lost jobs.

Where's the business community on ENDA? 

Individually, the vast majority of American businesses have already implemented protections similar to those provided by ENDA, including 96.6 percent of corporations on the Fortune 500 list (holdouts include Exxon Mobil and DISH). Some CEOs, like Apple's Tim Cook, have spoken out in favor of the bill. But business lobbying organizations like the Business Roundtable, National Federation of Independent Business, and Chamber of Commerce -- which serve as those companies' muscle in Washington and would be the ones to whip up Republican votes -- have declined to take a position.

Gay-rights advocates count their lack of outright opposition as a victory, gained through a business-friendly concession. "ENDA's congressional sponsors persuaded the business associations like the U.S. Chamber of Commerce to move from opposition to neutrality by including ENDA language that explicitly bars lawsuits brought under the 'disparate impact' theory," says Freedom to Work's legislative director Christian Berle, referring to a doctrine that makes companies liable for behavior that results in an unequal outcome, not just discrimination on the front end. That means ENDA would not create the same level of protections that exist for other kinds of historically disadvantaged minority groups.

What are its political chances?

Clearing the Senate is a big victory that didn't seen like a sure thing Monday morning, but the House is going to be a tough battle. Nonetheless, if Boehner did allow the bill to come to a vote, it might have a decent chance of passage: Research shows solid public majorities in favor of workplace protections for GLBT people across the country, and serious money is starting to flow toward conservative candidates who back gay rights, so the repercussions for a yes vote might not be as large.

Could President Obama just make it happen without Congressional action?

Not exactly. He could issue an executive order effectively making ENDA apply to all federal contractors, but has said that he'd prefer even that level of protection be achieved through the legislative process rather than presidential fiat.