D.C. Mayor Muriel Bowser says an estimated 80,000 residents could lose their health insurance if Congress repeals the Affordable Care Act. (Manuel Balce Ceneta/AP)

Republican proposals to repeal the Affordable Care Act could strip health insurance from tens of thousands of D.C. residents and cost the city more than $500 million a year, Mayor Muriel E. Bowser said in a letter to GOP congressional leaders Friday.

In a letter to House Majority Leader Kevin McCarthy (R-Calif.), Bowser urged caution as the new Congress pushes forward with plans to dismantle President Obama’s signature health-care law and replace it with a still-undefined alternative.

Bowser (D) told McCarthy that any Republican replacement for the ACA should ensure the same access to and quality of coverage people have now and guarantee that the city “is not placed at greater financial risk” because of new obligations to the uninsured.

“The repeal of all or part of the Affordable Care Act would disrupt our insurance market, impair insurers, and increase costs to the district and consumers,” Bowser said.

Kevin Harris, the mayor’s spokesman, said she opposes the repeal effort.

“Mayor Bowser believes the ACA has been a tremendous asset in helping District residents receive quality and affordable health care,” Harris said in an email. “Rather than going backwards with repeal, she believes we should be looking at ways to move forward and ensure affordable health-care coverage for every District resident.”

Bowser’s letter comes as state government officials across the country grapple with their financial and moral responsibilities to people who could lose their health coverage under a Republican overhaul of the insurance market.

New York Gov. Andrew M. Cuomo (D) asserted Wednesday that his state stood to lose $595 million this year in federal money that has been used to expand Medicaid.

This week, the GOP-dominated Congress set in motion plans to repeal the ACA, an action that would fulfill campaign promises by President-elect Donald Trump and reverse years of fruitless efforts to gut the law by its opponents on Capitol Hill.

Republican leaders say they will replace the law with a better system but have not specified what it would be.

The District — which offered its own version of universal health coverage to residents before the ACA’s enactment — could find itself scrambling to create and fund a plan that would help those left in the lurch by the law’s unraveling.

Many District residents have benefited from Obamacare — especially its option that allowed jurisdictions to expand Medicaid coverage. By the 2018 fiscal year, about 90,000 people in the nation’s capital would have health insurance because of the expansion, Bowser wrote in her letter, at an annual cost of $623 million, most of it picked up by the federal government.

The mayor said that many are also are taking advantage of another key component of the law — online clearinghouses, or “marketplaces,” where individuals and businesses can browse plans from competing insurance providers. Depending on income, individual customers can also qualify for government subsidies to help pay their insurance premiums.

Bowser said there are about 19,000 people who have policies through the individual marketplace and 60,000 covered under the small-business marketplace.

Harris said the figures cited in the mayor’s letter came from the city’s Department of Health Care Finance.