Mo. Rev. Stat. § 128.130

If any of the electors appointed under this chapter shall fail to attend at the seat of government by the hour of two o'clock in the afternoon of the day appointed by act of congress for their meeting, then the electors present may appoint other persons to act as electors in the place of those absent; and if there be a failure to elect, the electors attending at such time and place shall appoint some suitable person or persons, as the case may be, to fill the vacancy or vacancies existing by reason thereof.