Spain’s Iratxe García, who leads the group, has so far kept Spanish lawmakers in line. She faces a leadership vote on Oct. 21, though she is broadly expected to keep her post.

Should Sánchez lose his election, all the S&D’s largest national delegations — from Italy, Romania and France — would be in opposition at home. The only S&D parties still in government would be junior partners in Germany and Austria, alongside Denmark’s Social Democrats, whose hardline stance on migration limits their influence in the group.

Without a heavyweight prime minister from a large EU country to keep the S&D in line, von der Leyen would face greater pressure to make more concessions to lawmakers already frustrated by the center-right’s votes with the far right.

“If von der Leyen still delivers on S&D priorities, she will have the support of the group, and if that is not the case, then she will lose the support of the group,” Repasi said. A formal alliance between Spain’s EPP and far-right Vox, he added, “will drive us even more apart.”

The S&D would also have fewer “incentives” to compromise without Sánchez’s presence, the second senior lawmaker said, because the group would have less responsibility for maintaining stability in EU institutions.

“The institutional responsibility to guarantee support for the Commission and stability in the EU institutions will no longer be there,” the lawmaker said. A tougher line would be easier for the center-left “if the EPP continues striking deals with the far right, and the European Commission fails to offer a politically attractive agenda that progressives can sell to their voters.”