He does not have to resign if he leaves his own party. Of course he can join another party in the Knesset. The parties are a very fragile entity the system of election gave birth to the idea of coalition governments. These multi-party cabinets produced fragile regimes. The infrastructure of the Israeli politics therefore is detrimental to the building of a strong coalition cabinet.
Parties can easily disintegrate.
The party loyalty is shrinking. Indeed, the prestige of the Knesset is very low. There is a credibility gap between the masses and the Israeli political establishment. After almost 52 years, Israel finds itself on the verge of political turmoil. Israel cannot establish a strong, effective and functioning executive branch.