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Janković presented the idea at today's session of the Ljubljana city council, also calling on Prime Minister Miro Cerar and his Modern Centre Party (SMC) to join the initiative.
Erjavec signed the agreement and said at a press conference he held alongside Janković that a possible centre-right government would not be good for development of Slovenia.
The current public opinion polls suggest that this could be the case, but DeSUS would not like to participate in such a government, he added.
Erjavec sees the main problem in the fragmentation of the left, as there is a series of parties which can reach a certain result, but not good enough to form a centre-left government.
As DeSUS president fears a low turnout among the centre-left voters for this reason, he sees today's agreement as a trigger for mobilisation of voters.
Erjavec called on the Social Democrats (SD) and the SMC to join the bloc, saying that Janković was the only person capable of connecting parties on the left. He believes that a deal could also be made with the radical Left.
Janković added that a leftist government had to continue to govern Slovenia, as the country had achieved stability, peace and one of the highest economic growths in Europe in the last four years.
While labelling Erjavec as an experienced politician, he said that the party of Kamnik Mayor Marjan Šarec was also welcome.
Janković reiterated that he would not be running in the election with his party, but stressed that he would do everything in his power so that the next government was not led by SDS president Janez Janša.
Cerar, who attended the ceremonial city council session, said that the SMC was going to the election independently, but added that the party would keep an eye on the initiative to see how it would develop.
Left deputy group head Luka Mesec meanwhile said that it was ridiculous that "one of the two initiators of the so-called left bloc is a man who was in both Janša governments," referring to Erjavec.
"People who are forming anti-Janša coalitions ahead of elections are leftists only ahead of elections," he added.
