Israeli PM Benjamin Netanyahu's party will run alongside that of his ultra-nationalist Foreign Minister, Avigdor Lieberman, in January's election.
Mr Netanyahu said Likud and Yisrael Beitenu's joint list would create a stable government able to deal with the security and economic challenges ahead.
Mr Lieberman said he was confident the alliance could win the election, which has been called nine months early.
Opinion polls put the ring-wing list ahead of leftist and centrist parties.
But a survey published by Channel Two television suggested Likud and Yisrael Beitenu would win more seats in the Knesset as separate entities.
"Unifying lists usually shrinks them," wrote Nahum Barnea in a commentary for the biggest-selling newspaper, Yedioth Ahronoth.
"Anyone who did not tolerate Lieberman and voted for Netanyahu will think twice, and the same is true for those who did not tolerate Netanyahu and voted for Lieberman."