SACRAMENTO — California state lawmakers are expected as soon as Monday to put a state constitutional amendment on the ballot that would explicitly protect reproductive rights.

The amendment, which would go to California voters in November for approval, comes as states across the country react to the sweeping Supreme Court decision ending longstanding abortion protections.

At least 15 states and the District of Columbia affirmed or expanded abortion rights before Friday’s court shift, while roughly two dozen other states signaled that they would end or dramatically restrict access to the procedure.

Gov. Gavin Newsom of California has vowed to “fight like hell” to sustain abortion rights in the state. In the past year, Mr. Newsom has ramped up funding for abortion providers; offered tax breaks for companies seeking to move from states where abortion may be outlawed; and signed bills to protect abortion patients from privacy intrusions, insurance co-pays and threats, pressure or other attempts at “reproductive coercion.”

On Friday, Mr. Newsom signed a bill to shield California abortion providers from liability or prosecution related to out-of-state bans on abortions. He also announced an agreement with Gov. Kate Brown of Oregon and Gov. Jay Inslee of Washington to establish a West Coast abortion firewall that would protect providers and patients from the legal reach of other states.

Pending bills would authorize experienced nurse practitioners to perform first-trimester abortions without a physician’s supervision and create a state-administered fund to help underwrite travel expenses for the many women from abortion-ban states expected to come to California for an abortion.

The legislative blitz reflects California’s widespread support of a woman’s right to choose when and whether to bear children. A poll last year by the Public Policy Institute of California found that roughly four out of five California voters oppose the repeal of Roe v. Wade.

The state’s constitution already includes a right to privacy that has long underpinned abortion statutes. But after the Supreme Court leak in May, the state’s legislative leaders moved to make certain that abortion and contraception are explicitly protected. A similar constitutional amendment will go to Vermont voters in the fall, ensuring “personal reproductive liberty.”

California’s constitutional amendment “would make it undeniably clear that in California, abortion and contraception are health care and are a private matter between a patient and their medical provider,” said Toni G. Atkins, the president pro tempore of the State Senate, who directed a women’s health clinic in San Diego before entering public service.

The Senate has already passed the amendment with supermajority approval, and it needs to receive at least two-thirds support in the Assembly by this week to be placed on the ballot in November. The amendment does not require Mr. Newsom’s signature to reach the ballot, but he supports it.

Opponents include the California Catholic Conference, which criticized the proposal’s wording as “extreme” and so loose that it could be interpreted as allowing late-term abortions.

“The sad reality is that California already has some of the most accommodating abortion laws and services in the nation,” the conference said in a statement, vowing to muster “the state’s 12 million Catholics” in its fight against the amendment.