John Gonzales made the drive from his Napa home to San Francisco on Sunday to show support for the Affordable Care Act. For him — just like it was for hundreds who turned out at City Hall to protest President-elect Donald Trump’s pledge to undo the health care law — it was personal.

Three years ago, the 40-year-old engineer lost his job and was paying for medical insurance out of his own pocket when he learned that his son had an immune system deficiency. The 2-year-old would need a bone marrow transplant as well pricey medication, and their insurance company didn’t want to go forward with coverage.

But under President Obama’s signature legislation, which doesn’t allow insurers to drop customers because of health conditions, his son got the requisite surgery as well as the antirejection drugs that he takes to this day, Gonzales said.

“I don’t know how I would have done it” without Obamacare, said Gonzales, who now runs his own company. “My son’s basically cured, but he needs medication once a month, and it’s pretty expensive.”

Almost 2,000 people, including such notables as recent Rock and Roll Hall of Fame inductee Joan Baez, rallied Sunday afternoon in Civic Center Plaza behind the benefits of the Affordable Care Act. The demonstration was among several coordinated nationwide, from Los Angeles to Detroit, to fend off a Republican-led dismantling of the 2010 law, one of outgoing President Obama’s signature achievements.

On Friday, a week before Trump takes office, Congress voted to begin the repeal process. While the legislation has added millions of Americans to the health care rolls, critics have decried its costs.

Headlining Sunday’s show of speakers, House Minority Leader Nancy Pelosi, D-San Francisco, panned her Republican counterparts for acting to take away people’s health insurance.

“Their plan is to make America sick again,” she said to loud cheers.

Signs with such slogans as “Trump makes me sick” and “Health care is a human right” were ubiquitous in the crowd.

While Trump and his congressional allies have said they will come up with a policy to replace Obamacare — one that is “better,” according to the president-elect — they have yet to offer details.

“The Republicans don’t have a plan,” Pelosi said. “They want to cut and run.”

Pelosi was joined at the podium by several in California’s Congressional delegation, including Rep. Anna Eshoo, D-Palo Alto, and Rep. Mike Thompson, D-St. Helena, as well as San Francisco Mayor Ed Lee.

“A repeal vote will start us down the path toward the road to chaos,” Lee said. “It means turning our backs on the most vulnerable.”

Perhaps the most popular to take to the makeshift stage Sunday was Baez, who performed an a cappella version of the classic hymn “Swing Low, Sweet Chariot.”

With an afternoon full of personal and passionate calls to save Obamacare, and no opposition in the vicinity, Baez was figuratively singing to the choir. California’s widespread support of the law isn’t likely to be enough to stop what will amount to a procedural budget vote in Washington soon to can the program, experts say.

The Affordable Care Act is credited with expanding health coverage to more than 20 million people, including about 4 million in California.

But it’s had problems. Premiums and co-payments for many of the newly insured are rising, and subsidies to help them are testing the budgets of states and the federal government, which are having to cough up more funding to insurance companies.

The process of developing an alternative policy, as Republicans have promised, is complicated by the fact that popular parts of the Affordable Care Act are paid for by its less favorable components, like special taxes and a mandate that everyone must buy insurance.

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander