The internal dissent that has long wracked the 137-year-old Socialists (PSOE) has burst into the open after it took a drubbing in two weekend regional polls, on top of historically low results in December general elections and in a repeat June vote.
As Spain's political paralysis drags on following two inconclusive general elections, many within the PSOE want the party to let a right-wing coalition government through parliament by abstaining in the necessary vote of confidence, and go into opposition where it can build up strength again.
But instead the Socialists voted against such a government earlier this month, prompting it to fail, and Sanchez is now trying to form his own coalition with Podemos, the anti-austerity party that wants to replace it as the country's main left-wing force.
"He told me he would go into opposition, that he would not attempt any alternative government," Gonzalez told the Cadena Ser radio, adding Sanchez had made those comments on June 29, just days after repeat general elections.
"I feel frustrated... as if I had been tricked."
As such, many high-ranking members of the party are preparing for a showdown with Sanchez on Saturday, when they gather for talks, and will try and pressure him to quit.
But in a statement reacting to Gonzalez's comments, Sanchez said that voting against a coalition government led by acting Prime Minister Mariano Rajoy was not solely his idea, but also that of the party's leadership which had "set the Socialists' position."
Gonzalez is not the only high-profile Socialist to have come out against Sanchez.
Several of the so-called "party barons" - or regional presidents - have also criticised him.
"The PSOE is killing the PSOE," wrote left-wing online daily eldiario.es in an opinion piece.
"A bloody and shameless spectacle in which the audience appears not to count much... And even less the rights and needs of citizens."