When any act before the Legislature proposes an amendment to the Constitution and proposes calling a special election for submission of such proposal, and such act on final reading receives a favorable vote of less than four-fifths of the members elected to the Legislature, the act shall then be amended to provide for submission of such proposition at the next election of members of the Legislature, and again be placed on final reading. If such act receives a favorable vote of three-fifths of the members elected to the Legislature, the proposition shall be submitted to the electors as provided therein.