Big news: The Senate will vote on Monday on the Employment Non-Discrimination Act, which would bar discrimination in hiring and other job-related decisions based on sexual orientation for all but the smallest businesses.

Can it secure the 60 votes it needs to break a GOP filibuster? Right now, one of the last remaining obstacles to passage may be an argument over “religious liberty.”

Here’s the situation. ENDA has 59 votes right now — the 55 Dems (including Cory Booker), Republicans Susan Collins and Mark Kirk, who co-sponsored the bill, and Orrin Hatch and Lisa Murkowski, who voted for it in committee. Gay rights advocates and Dems — led by Jeff Merkley — are aggressively lobbying four more Republicans: Rob Portman, Kelly Ayotte, Dean Heller, and Pat Toomey.

Conversations are ongoing with all of them, though Portman seems to be the most engaged, a source familiar with discussions says. If Portman comes on board, it’s possible or even likely (though hardly certain) that the remaining three Republicans will come along too — and proponents are hoping to get well over 60 votes, possibly as high as 65.

Here’s where “religious liberty” comes in. One of the last concerns Portman has is ENDA’s “religious liberty” protections. The bill has an exemption for religious organizations, but employers cannot cite religious reasons for hiring decisions. Portman told the Ohio press he’s still struggling with this piece of the puzzle.

But Democrats and advocates point out the religious exemption is already quite broad. The ENDA legislation exempts churches and religious organizations, including educational institutions — the same group of institutions that Title VII of the Civil Rights Act exempts from strictures against discrimination.

But ENDA contains a broader basis for allowing discrimination. Under Title 7, these institutions need a religious rationale for discrimination. Under ENDA, they just have to be religious institutions to discriminate, and don’t need to cite any rationale. Some on the left, such as the ACLU, are concerned the exemption isn’t narrow enough, fearing it could give institutions a “blank check” to discriminate.

“While we would have preferred a narrower religious exemption, we understand that the legislative process requires give and take,” Fred Sainz, a vice president at Human Rights Campaign, tells me. “This bill respects religious organizations and their fundamental beliefs and tenets in a way that’s substantially broader than the existing exemption for religious organizations in Title VII.”

It seems reasonable to expect that the final outcome could resemble the endgame in the battle over Don’t Ask Don’t Tell. After weeks of behind the scenes wrangling, repeal passed by a bipartisan margin, with half a dozen Republicans breaking in favor of ending legalized discrimination.

This is a similar vote: With the culture rapidly shifting on gay rights, you may see more Republicans come out and try to get on the right side of history before it’s too late.

Of course, then it’s up to the House of Representatives, where the GOP majority doesn’t appear all that concerned with getting on the right side of history.