After taking office in June, Mr. Sánchez sought to renew the political dialogue with the governing pro-independence politicians in Catalonia, after their botched attempt to secede in 2017, a move that was unconstitutional.

But Mr. Sánchez got nowhere and has instead been accused of treason by opposition politicians for trying to negotiate with Quim Torra, the separatist leader of Catalonia, as well as for allying himself with some Basque nationalist lawmakers.

“The question is whether we want a government in the hands of those who want to destroy Spain,” Albert Rivera, the leader of Ciudadanos, said last week. “I want a government without separatists.”

Mr. Sánchez recently said that he would not allow an independence referendum in Catalonia, but he has avoided the issue of whether he may eventually use his executive powers to pardon former separatist leaders who are on trial in the Supreme Court, should they be convicted of rebellion and other crimes over the 2017 secession attempt.

• In addition, one of Mr. Sánchez’s first pledges upon taking office was to exhume the remains of the dictator Franco, who died in 1975 and was buried in an underground basilica. That exhumation project has run into problems, delaying Franco’s reburial at least until June. Mr. Sánchez has instead focused on the emergence of Vox to warn voters that a right-wing coalition could risk returning Spain to the ultranationalism of the Franco era.

The prime minister recently accused the Popular Party and Ciudadanos of “embracing without a blush the arguments of this extreme right that has always existed in our country.”