When Jerusalem was demolished by Nebuchadnezzar, the kingdom was destroyed, and no longer were there hereditary successions to the sovereignty as formerly; at that time, moreover, being out of power, the posterity of David were living in captivity. But, when the followers of Salathiel and Zerubbabel returned, they led the people more democratically, transferring the rule henceforth to the priesthood because of the intermingling of the priestly and royal tribes.
Source: LETTER 236.2