SAN ANTONIO — Gay rights advocates and religious conservatives were expected today to pack a City Council meeting for a vote on nondiscrimination protections that are already in place elsewhere in Texas but that have drawn rebuke from big-name Republicans.

The proposal would amend San Antonio’s nondiscrimination code to include sexual orientation and gender identity, and thereby add the nation’s seventh-largest city to a list of nearly 180 other U.S. cities with similar ordinances, according to the Human Rights Campaign.

What began as a local issue has mushroomed into a heated debate attracting GOP heavyweights such as U.S. Sen. Ted Cruz and Texas Attorney General Greg Abbott, who is the early favorite in the 2014 governor’s race.

Abbott warned the city that passing the ordinance would invite a lawsuit, though a carefully worded letter to Democratic Mayor Julian Castro on Wednesday did not say that challenge would come from the state.

“Forced compliance with the proposed ordinance does not promote diversity. It tramples it,” Abbott said. “And when the diversity being trampled is religious diversity, the Constitution must be reckoned with.”

Gay rights victories in Texas haven’t come at the Capitol but at city hall. While nondiscrimination bills in the Legislature have languished, Houston has a lesbian mayor and Austin offers health benefits for same-sex couples.

The conservative pushback in San Antonio is notable coming on the turf of Castro, a rising star who delivered the keynote address at the Democratic National Convention last year. It also sprouts a weed in Castro’s narrative that San Antonio embraces political values that he says are spreading statewide and will eventually turn Texas blue.

Castro said the opposition to what he called an “overdue” and level-headed amendment was disappointing.

“These days, unfortunately, it’s campaign season,” Castro said. “What else would you expect?”

For Republicans, who hold every statewide office in Texas and mock predictions that a Democratic resurgence is on the horizon, the San Antonio proposal has rallied supporters and become an early stakeout ahead of the 2014 primaries.

Hundreds of congregants from black and Latino churches have rallied against the ordinance on the steps of City Hall.