WARREN, Mich. — Thousands of people showed up in freezing temperatures on Sunday in Michigan to hear Sen. Bernie Sanders denounce Republican efforts to repeal President Barack Obama’s health care law, one of dozens of rallies Democrats staged across the country to highlight opposition.

Labor unions were a strong presence at the demonstration in a parking lot at Macomb Community College in the Detroit suburb of Warren, where some people carried signs saying “Save our Health Care.”

Lisa Bible, 55, of Bancroft, Michigan, said she has an autoimmune disease and high cholesterol. She said the existing law has been an answer to her and her husband’s prayers, but she worries that if it’s repealed her family may get stuck with her medical bills.

“I’m going to get really sick and my life will be at risk,” said Bible, an online antique dealer.

President-elect Donald Trump has vowed to overturn and replace the Affordable Care Act, and majority Republicans in Congress this week began the process of repealing it using a budget maneuver that requires a bare majority in the Senate.

“This is the wealthiest country in the history of the world. It is time we got our national priorities right,” Sanders told the Michigan rally.

The law has delivered health coverage to about 20 million people but is saddled with problems such as rapidly rising premiums and large co-payments.

Britt Waligorski, 31, a health care administrator for a dental practice, said she didn’t get health insurance through work but has been covered through the health law for three years. While the premiums have gone up, she said she is concerned that services for women will be taken away if it is repealed.

“It’s done a lot for women for their annual checkups, for mammograms — women’s health in general. If this gets repealed, we’re going to go back to the old days when that’s not covered,” she said.

At the rally in San Francisco, Silvia Pena, a 45-year-old nanny, said she had never had held insurance until she enrolled in the Affordable Care Act six years ago.

“I don’t have health issues, but you can need insurance any time. We should all have access to health services,” said Pena, who held a sign that read “It’s our right and our body. Keep Planned Parenthood and Obamacare.”

About 2,000 people cheered and held rainbow and American flags and signs that read “Don’t Make America Sick Again” and “Health Care For All” at the rally. Patients who survived cancer, strokes and other health difficulties are sharing stories on how they benefited from the mandated health insurance.

In Los Angeles, organizers of the rally outside the LA County/USC Medical Center warned that a repeal of the law without a replacement will throw the state’s health care system into chaos and strip coverage from 5 million Californians.

Rallies in other cities in support of the health law also were well attended. Police estimated about 600 people showed up in Portland, Maine. Hundreds also attended events in Newark, New Jersey, Johnston, Rhode Island, Richmond, Virginia and Boston.

The health law has provided subsidies and Medicaid coverage for millions who don’t get insurance at work. It has required insurers to cover certain services such as family planning and people who are already ill, and has placed limits on the amount that the sick and elderly can be billed for health care.

Republicans want to end the fines that enforce the requirement that many individuals buy coverage and that larger companies provide it to workers. But they face internal disagreements on how to pay for any replacement and how to protect consumers and insurers during a long phase-in of an alternative.