Mo. Rev. Stat. § 115.095: Judge failing to appear, temporary judge to be appointed, how.

If any judge fails to act or to appear by the time fixed by law for the opening of the polls, the election authority shall be notified immediately by an election judge. The election authority or the election judges present in the polling place shall appoint another judge from the same political party as the judge failing to act or to appear. If the election judges elect a qualified temporary judge, such judge shall have full authority to act as judge for the election, except that such judge may be removed at any time by the election authority and replaced with another qualified judge from the same political party as the removed judge. Any judge selected pursuant to this section shall be selected to ensure that no political party shall have a majority of judges at any polling place and that each major political party has at least one judge serving at the polling place.