Two Israeli PM Benjamin Netanyahu leaders in April elections announced a centrist alliance in a bid to defeat him.
Chief Ex-army Benny Gantz and political center Yair Lapid agreed to rotate as prime minister if they win.
Separately, Mr. Netanyahu has set up an alliance with many distant parties to test the votes.
His liking party in Likud led to polls despite investigating allegations of corruption against him.
The Israeli lawyer is generally due to a decision whether Mr. Netanyahu, who refuses allegations, should be charged in the coming weeks.
Gantz, the former leader of the Israel Defense Forces (IDF), and TV-journalist-politician replaced Yair Lapid expressed their intention to run in a joint centrist ticket on Wednesday, Thursday before Thursday to submit candidates' list for April 9.
They also include former Defense Minister Moshe Yaalon and former military chief of the Night Ashkenazi.
The agreement with the center of the land shows that race is more competitive and a challenge to the decade-long premier of Mr Netanyahu, BBC's correspondent Tom Bateman told.
The Resilience party of Mr. Gantz said in a statement that "decided to establish a joint list of new Israeli party leaders", and swore to reconcile Israel's society.
If successful at the polls, Mr. Gantz agreed to hold office as prime minister for the first two-and-a-half years, before Mr Lapid took on.
Where is Netanyahu staying?
The Likud party of Mr Netanyahu will win the most parliamentary seats and become a position to form another governing coalition of nationalist and religious parties.
All governments in Israel are coalitions because of the system of proportional representation of the country, meaning one party is unlikely to control alone
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On Wednesday, Mr Netanyahu helped arrange a merger of far-right Jewish Home and ultra-parties, in a bid to boost the total number of seats held of parties entitled to the partnership after the polls.
As part of an alliance with his Likud party, he agreed to allocate two postrooms for the Jewish Home party, as long as it did not agree with the merger.
However, his critics expressed the move to join the forces of Jewish Power, which included the disciples of late anti-Arab rabbis Meir Kahane.
The US-born rabbi pushed a fundamentalist group to the right in the 1
Mr Netanyahu's campaign is surrounded by the possibility of a general lawyer receiving police recommendations to hear him hesitant in making fraud and bribery in three cases.
The prime minister is believing to stay in the office what the decision of the attorney general is.
If he wins, he will be the longest-serving leader of Israel, taking the title from David Ben-Gurion.