"I aspire to be elected prime minister, but if I'm not, I will head the opposition," she said at a meeting attended by Labor Knesset members and candidates.
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After the meeting, Yachimovich told reporters the decision came in response to the radical turn that Likud's positions have taken recently.
Netanyahu and Yachimovich. (Photo: Yaron Brener, AFP)
"Netanyahu is planning a social and economic hell," she said. "There radicalization is evident in every aspect. (We're seeing) an economical jungle, and deep contempt for the rule of law. I can't imagine what will happen to the rule of law if Netanyahu is reelected."
She also blasted the prime minister for embracing former Foreign Minister Avigdor Lieberman, whom she called a "corrupt man."
Yachimovich has previosly avoided coming out with a definite statement about her post-election plans with regards to joining the coalition, despite pressure. On Thursday she said that the changes the Likud has undergone have wiped any chance that she would work alongside Netanyahu.
"I refused to reject the possibility of joining Netanyahu. I made sure to say that it was unlikely, but I didn't reject it," she said. "But this isn’t the Likud that we all know."
Earlier in the day, sources within her party branded the current administration as belonging to the "radial-Right" and vowed to protect the democratic character of the state.
"Given the evident radicalization in the positions held by the Likud-Yisrael Beiteinu on all issues of national importance, including the economy, social matters, state policy, defense, rule of law and democracy, the Labor Party remains the only alternative to the current government and therefore will not join Netanyahu's coalition," one source said.
Moran Azulay is a Ynet and Yedioth Ahronoth correspondent
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