Herzog, Livni, Lapid, and the rest of the Israeli left seem to think Israel should always bow to the dictates of the US president, especially when he is hostile to Israel.
They seem to have prioritized protecting Zero, who they think is (or should be) the PM of Israel over winning the election.
Zero is incredibly unpopular in Israel and has been since he first took office, so by presenting a united front with him they know they are guaranteeing a Netanyahu victory.
That’s how I read the situation, anyway.
An interesting theory.