Mo. Const. art. X, § 22

(a) Counties and other political subdivisions are hereby prohibited from levying any tax, license or fees, not authorized by law, charter or self-enforcing provisions of the constitution when this section is adopted or from increasing the current levy of an existing tax, license or fees, above that current levy authorized by law or charter when this section is adopted without the approval of the required majority of the qualified voters of that county or other political subdivision voting thereon. If the definition of the base of an existing tax, license or fees, is broadened, the maximum authorized current levy of taxation on the new base in each county or other political subdivision shall be reduced to yield the same estimated gross revenue as on the prior base. If the assessed valuation of property as finally equalized, excluding the value of new construction and improvements, increases by a larger percentage than the increase in the general price level from the previous year, the maximum authorized current levy applied thereto in each county or other political subdivision shall be reduced to yield the same gross revenue from existing property, adjusted for changes in the general price level, as could have been collected at the existing authorized levy on the prior assessed value.

(b) The limitations of this section shall not apply to taxes imposed for the payment of principal and interest on bonds or other evidence of indebtedness or for the payment of assessments on contract obligations in anticipation of which bonds are issued which were authorized prior to the effective date of this section.