Spain’s leftwing Podemos party has ended days of uncertainty by agreeing to an awkward, last-minute deal to run in next month’s snap general election alongside Sumar, a new leftist alliance led by the communist labour minister, Yolanda Díaz. Although the deal, announced hours before the deadline for registering coalitions expired, is intended to head off a split vote and to galvanise leftwing voters against the conservative People’s party (PP), which is leading the polls, there are enduring differences and tensions between Podemos and Sumar. Chief among them is the candidacy of Podemos’s Irene Montero, who serves as equality minister in the socialist-led coalition government, and who was the main proponent and defender of the botched and deeply unpopular sexual consent law under which more than 100 sex offenders were inadvertently released from jail. Speaking after Podemos members voted overwhelmingly in favour of allowing the party leadership to negotiate a pact with Sumar, Podemos’s leader, Ione Belarra, confirmed that a deal had been reached. But she pushed back on Sumar’s opposition to Montero. “I’ll say this very clearly,” Belarra said on Friday afternoon. “We will run with Sumar in the general election. That decision has been taken. However, I also feel I have a moral obligation to broach some important questions. The first is that Yolanda Díaz’s negotiating team has told us that the presence of Irene Montero in the team that will run in the next general election is an insurmountable obstacle when it comes to reaching a unified agreement.” The Podemos leader said she was “deeply saddened” at Díaz’s insistence that the deal rested on the exclusion of Montero, whom she described as “a colleague who has taken feminist transformation further than anyone before her in this country”. She said the equality minister had brought in abortion reforms as well as new laws on sexual consent and trans rights. Belarra said that while Montero had said she was happy for her party to decide her future, Podemos’s leadership felt that removing her would be an injustice and an “enormous political error”. Belarra also said her party, which had disastrous results in the municipal and regional polls last month that led the socialist prime minister, Pedro Sánchez, to call the snap election, was worried the deal could leave it without any seats in congress. While she said Podemos’s signature on the coalition agreement was “guaranteed”, she called for a “fair agreement” with Sumar. Montero, whose partner is the former Podemos leader Pablo Iglesias, has faced a huge backlash over the sexual consent reforms. The only-yes-means-yes law, introduced by the coalition government after the fury and revulsion that followed the “wolfpack” gang-rape in Pamplona seven years ago, made sexual consent a key factor in sexual assault cases. But it also revised the scope of potential minimum and maximum prison sentences, inadvertently allowing some convicted sex offenders to have their sentence reduced on appeal. To date, 1,127 offenders have had their sentences cut and 115 have been granted early release. Despite warnings from lawyers and criticisms from political opponents, Montero has continued to advance and defend the law. Earlier this year, Sánchez apologised for the changes and asked for the forgiveness of victims. The prime minister announced the early election – which had originally been scheduled for December – hours after the PP scored an emphatic win in the local and regional elections. Despite the PP’s victory, the polls suggest it is not on course to win an absolute majority in the general election on 23 July and could have to rely on the support of the far-right Vox party. Sánchez is gambling on the notion that the prospect of such a government will help mobilise leftwing voters across Spain.