United States politics are dominated by two major political parties , the Democratic and Republican parties , who choose their presidential candidates in nominating conventions attended by delegates from states . State law determines how each parties ' delegates are chosen in that state including by either a primary election or a caucus and on what date those contests are held . State governments are free to choose whichever date they want for their states ' primary or caucus . With the broadened use of the modern presidential primary system ( following the chaotic 1968 Democratic National Convention in Chicago ) , states have tried to increase their influence in the nomination process . One tactic has been to create geographic blocs to encourage candidates to spend time in a region
1968 Democratic National Convention
1968 Democratic National Convention
Super Tuesday