Complex sentence: Washington's campaigns took place prior to the 1804 ratification of the Twelfth Amendment, which instituted the current system in which each member of the Electoral College casts one vote for president and one vote for vice president.
Split into simpler sentences: Washington's campaigns took place prior to the 1804 ratification of the Twelfth Amendment, which changed some of the procedures of the Electoral College. The Twelfth Amendment instituted the current system in which each elector casts one vote for president and one vote for vice president.