Spain’s highest court convicted a former top official of the country’s governing Socialist Party on Monday and sentenced him to 24 years in prison for bribery and other acts of corruption. The verdict has heaped more pressure on Prime Minister Pedro Sánchez as investigations swirl around his former allies and family.

José Luis Ábalos used to be a top official in Mr. Sánchez’ Socialist Party and an infrastructure minister with control of billions of euros of the Spanish budget. The court found him guilty of criminal organization, bribery, embezzlement and influence peddling. Mr. Ábalos’s top lieutenant, a former bouncer and driver turned government fixer, Koldo García, was also found guilty of corruption charges, including accepting envelopes filled with bribes for the minister. He received a sentence of 19 years. Both men had pleaded not guilty, and their lawyers did not immediately return a request for comment.

The conviction is a serious blow to Mr. Sánchez, a left-leaning leader who has cultivated an image abroad as a liberal darling willing to stand up to President Trump and the far right but who is swamped at home with associates and family members facing corruption accusations.

A decade ago, as Mr. Sánchez embarked on a primary campaign that accelerated his remarkable ascent to the top of the Spanish government, he drove around Spain in his black Peugeot with Mr. Ábalos and Mr. García. The journey became central to Mr. Sánchez’ political mythology. With the convictions, it has turned into a political nightmare.

Investigations or prosecutions have also begun against:

• Begoña Gómez, the wife of Mr. Sánchez, who was ordered on Saturday to surrender her passport and stand trial on charges of corruption and influence peddling in a case that has been under investigation since 2024.

• David Sánchez, Mr. Sánchez’ brother, who went on trial early this month over allegations that he received a patronage job.

• José Luis Rodríguez Zapatero, a former prime minister and ally of Mr. Sánchez, who was accused in May of receiving kickbacks of up to roughly $2.4 million for, among other activities, helping persuade the government to bail out an airline.

• The Socialist Party, the headquarters of which were raided last month by police officers looking for evidence of a mudslinging campaign led by Mr. Ábalos’s replacement against judges investigating Mr. Sánchez’ family.

Mr. Sánchez has called the cases against his wife and brother, who have denied the accusations, politically motivated, and many judicial experts and political analysts have voiced doubts about those trials. He has also stood by Mr. Zapatero, who has maintained his innocence. But Mr. Sánchez had desperately sought to distance himself from Mr. Ábalos.

Taken together, the trials have given Mr. Sánchez’ conservative opponents significant ammunition.

“Spanish democracy is in a critical situation,” Alberto Núñez Feijóo, the leader of the Popular Party, Spain’s main conservative opposition party, said on Monday. He called Mr. Ábalos “Mr. Sánchez’ No. 2” and underlined the irony of Mr. Ábalos’s having given a now-infamous speech in Parliament in 2018, when the previous conservative government faced corruption scandals. That speech introduced a motion of no confidence and paved the way for Mr. Sánchez’ rise to power.

Now, Mr. Feijóo said, all the scandals around Mr. Sánchez were “unbearable and incompatible” with “a normal democracy” and it was his time to go.

Mr. Sánchez has denied any involvement in any corruption. On Monday, Mr. Sánchez’ office said he had no immediate comment. The Socialist Party said in a statement that “justice has spoken, and its rulings must be respected.” It added that, unlike the Popular Party, it had “zero tolerance for corruption.”

In Mr. Ábalos’s case, the court’s three judges ruled in a unanimous decision that he had received monthly kickbacks linked to the awarding of a government contract to provide masks during the coronavirus pandemic. The court ruled that Mr. Ábalos also received an apartment for his former mistress and sweetheart deals on other real estate.

“These are actions that undermine the democratic architecture of our social and democratic state under the rule of law,” the court said in its ruling. In a separate statement, the court added that there had been a “severe erosion of public trust.”

For weeks, the often-televised trial riveted Spain and filled the front pages of conservative newspapers. The defendants sat together in court, sometimes with their heads in their hands, as prosecutors and witnesses disclosed damning and sometimes humiliatingly tawdry evidence, including when Mr. Koldo’s ex-wife testified in unflattering terms about Mr. Ábalos’s ex-wife.

When officials initially raided the home of Mr. Ábalos last year, they said they found an external hard drive, containing possible evidence, hidden in the pants of a woman who had worked as a pornographic actress and whom Mr. Ábalos had asked to walk his dog, according to a police report reviewed by The New York Times.

Carlos Barragán contributed reporting.