WASHINGTON — The Senate on Wednesday adopted stronger protections for religious institutions in a historic gay rights bill that secured the conditional support of a key Republican who was the party’s presidential nominee in 2008.

On a quick, voice vot, the Senate approved an amendment from Republican Sens. Rob Portman of Ohio and Kelly Ayotte of New Hampshire that would prevent federal, state and local governments from retaliating against religious groups that are exempt from the law. The overall bill would prohibit workplace discrimination against gay, bisexual and transgender Americans.

The vote set the stage for passage of the bill on Thursday when all 55 senators in the Democratic majority and a number of Republican senators are expected to back the measure.

Asked if he would vote for the bill, five-term Sen. John McCain, R-Ariz., who twice sought the presidency, said, “yes,” then added in a brief interview, “if we get the amendments worked out.”

McCain was a co-sponsor of the Portman-Ayotte amendment. The Senate plans to vote on Thursday on an amendment by Sen. Pat Toomey, R-Pa., to expand the number of groups that are covered under the religious exemption.