McCaskill is running against Josh Hawley, the young challenger backed by President Donald Trump. Hawley, the 38-year-old, Ivy-league educated state attorney general, says the 65-year-old McCaskill is too liberal for Missouri. Trump won the state by 19 points in 2016.
McCaskill campaigned as a moderate and focused on health care issues. Republicans badly want to defeat McCaskill after she survived in 2012 when her opponent made a major gaffe.
Missouri voters also will face ballot measures Tuesday to raise the gas tax and a major change to drawing district boundaries after the 2020 census.