Austin Beutner, a Los Angeles civic and business leader who guided the city’s public school system through the coronavirus pandemic, is expected to announce on Monday that he is joining the 2026 race for mayor, posing the first serious challenge to the re-election of Mayor Karen Bass.

The anticipated entry of Mr. Beutner, 65, comes as Los Angeles has reeled under a yearlong cascade of troubles, including wildfires that exacerbated a daunting budget shortfall, as well as immigration crackdowns and National Guard deployments by the Trump administration.

But Mr. Beutner’s interest also underscores the political vulnerability of Ms. Bass, whose approval ratings plummeted this year after it emerged that she had been out of the country in January when wildfires ignited in Pacific Palisades, one of the city’s wealthiest enclaves. Only recently has the mayor’s popularity begun to rebound, a product of her public resistance to federal immigration raids.

In Mr. Beutner, Ms. Bass would face a former supporter whose appeal in Los Angeles, an overwhelmingly Democratic city, could reach beyond that of her opponent in 2022, Rick Caruso, a billionaire developer and a former Republican.

“I voted for Karen Bass last time — we all had hopes,” Mr. Beutner, a longtime donor and a supporter of liberal candidates and causes, said in an interview this week. “She did good work in the Legislature, good work in Congress. But the job of a mayor is a different job, and L.A. is adrift. Ultimately, that’s a function of leadership.”