Mo. Rev. Stat. § 290.560

Whenever there is a period of excessive unemployment in this state, every person who is charged with the duty, either by law or contract, of constructing or building any public works project or improvement for the state or any political subdivision, municipal corporation or other governmental unit thereof shall employ only Missouri laborers and laborers from nonrestrictive states on such project or improvement, and every contract let by any such person shall contain a provision requiring that such labor be used, except that other laborers may be used when Missouri laborers or laborers from nonrestrictive states are not available, or are incapable of performing the particular type of work involved, if so certified by the contractor and approved by the contracting officer.