In a Facebook post, he explained that the decision was made in order to better the chances of replacing Prime Minister Benjamin Netanyahu in the upcoming elections.
"The Israeli Left has lost its way," Shelah wrote, "leading to a loss of many votes to the far-right wing."
In order to prevent the loss of votes to the Right, Shelah believes that the Israeli Left needs a "clear ideology with political capability," which he has been trying to achieve with the establishment of the Tnufa party.
However, referring to the failed merger talks with Labor Party leader Merav Michaeli, Shelah wrote that despite his efforts, a merger that would create a joint and established opposition to the dominant Right, was not possible.
"Under these circumstances, I must uphold my promise to prevent the political camp trying to replace Netanyahu from losing more votes," Shelah wrote. "Tnufa will not be running in the upcoming elections, and I won't be either," he concluded, promising to remain active in the effort to replace Netanyahu.