But signs of the volatility of his coalition showed in early 2019, when Catalan lawmakers withdrew their support. Mr. Sánchez called an election and stayed on as a caretaker, but Spain endured months of political uncertainty until his re-formed coalition’s victory in January 2020.

In the ensuing years, the Covid pandemic hit Spain hard but Mr. Sánchez earned kudos from Brussels for his stewardship of relief funds, the Spanish economy improved and he sought a larger footprint in Brussels.

“We won’t be passive actors in the European debate,” Mr. Sánchez said at a business leader event in 2019. “We will be at the vanguard.”

But all along, the cracks in his coalition became more visible, and Spanish voters noticed. They also seemed to have tired of Mr. Sánchez himself, who as conservatives sought to nationalize the local races, making them a referendum on Madrid, became a drag on his party’s candidates even in apparent strongholds like Seville, where the popular mayor lost.