Spain’s prime minister, Pedro Sánchez, said late Wednesday that he was considering resigning after a judge opened an investigation into whether Mr. Sánchez’s wife had abused her position to help friends win public contracts.

The development stunned Spain and threw the political future of perhaps Europe’s most prominent progressive leader into doubt only months after he defied widespread expectations by putting together a fractious coalition and securing a second term in power.

“I need to stop and think,” Mr. Sánchez wrote in a long letter published on his X social media account on Wednesday evening. He canceled all political engagements until Monday to decide, he said, whether he “should continue to lead the government or renounce this honor.”

Recently, Mr. Sánchez had seemed to overcome another significant obstacle by assuring that the Catalan independent movement would support his coalition, making his second term in government seem sturdy.

But all that changed on Wednesday morning, when a judge responded to a formal complaint by a far-right group, Manos Limpias, and ordered an investigation into evidence against the prime minister’s wife, Begoña Gomez, for alleged influence peddling. Further details were not immediately available.

In the long letter published on X, Mr. Sánchez argued that the accusations were motivated by his political opponents, the People’s Party, or PP, and Vox.

In a post on X, the PP party responded, “His problem is not political, it’s judicial and is responsible for an episode that stains the international image of our country.”

“This is an operation of harassment and demolition by land, sea and air, to try to make me weaken politically and personally by attacking my wife,” Mr. Sánchez said.

He added that he would hold a news conference on Monday to inform the country of his decision.