California and 15 other states on Tuesday denounced a Trump administration proposal to let private businesses holding federal contracts use their owners’ religious views to justify employment practices, such as refusing to hire lesbians and gays.

“This is 2019, not 1920. No one should fear losing their job because of whom they love,” state Attorney General Xavier Becerra said in a statement accompanying a letter urging the U.S. Labor Department to withdraw the proposed rules. “The Trump administration is paving the way for federal contractors to openly discriminate against the LGBTQ community and anyone else who might not conform to their personal beliefs.”

The states said the change would weaken civil rights protections for federal contractors’ employees, who make up more than 20% of the private work force in the United States.

The proposal is the latest in a series of regulations that would broaden religious exemptions to federal or state antidiscrimination laws. The administration has sought to allow employers to deny birth-control coverage to women for religious or moral reasons, and is proposing to allow medical staff, including ambulance drivers and receptionists, to cite religious or moral objections in refusing to provide service to lesbians and gays.

The U.S. Supreme Court is preparing to consider, in the term that starts next month, whether federal sex-discrimination laws apply to lesbians, gays and transgender people. Nearly half the states, including California, provide such protections in their own laws.

The Trump administration’s proposal, announced last month, would expand an exemption issued in 1965 that allows expressly religious institutions, such as churches, mosques and religious universities, to prefer members of their own faith when hiring under federal contracts. Some courts have interpreted that exemption broadly to let the institutions make hiring decisions based on their religious beliefs, but it applies only to religious entities.

The regulations would extend the exemption to private, for-profit businesses with federal contracts, defining them as religious institutions based on their owners’ beliefs. They could then make hiring decisions for faith-based or moral reasons without liability under civil rights laws.

The change “helps to ensure the civil rights of religious employers are protected,” Patrick Pizzella, the acting secretary of labor, said when the proposal was announced. The White House insisted in a statement that the regulation would “responsibly protect religious freedom and members of the LGBTQ community from discrimination.”

But the American Civil Liberties Union said the rule would “license taxpayer-funded discrimination in the name of religion.” The ACLU also said the proposal could allow a contractor, for religious reasons, to fire a woman who was pregnant and unmarried.

The rule “would invite virtually any employer to self-designate as religious,” state officials, led by Becerra and Pennsylvania’s attorney general, said Tuesday in their letter to the Labor Department.

A contractor’s employees, who have a variety of spiritual beliefs, “will be forced to ascribe to their employer’s religious tenets” to protect their jobs if the employer claims a religious exemption, the letter said.

“The proposed rule will harm workers by opening the door to employment discrimination based on impermissible factors,” the state officials said. “Workers, families, taxpayers and states ... would be forced to bear the heavy costs.”

Besides California and Pennsylvania, the signers included attorneys general from Connecticut, Delaware, Hawaii, Illinois, Maryland, Massachusetts, Michigan, Minnesota, Nevada, New Jersey, New York, North Carolina, Vermont, Washington and the District of Columbia.

Bob Egelko is a San Francisco Chronicle staff writer. Email: begelko@sfchronicle.com Twitter: @BobEgelko