Coattail effect

The coattail effect is the idea that voters who support a strong presidential or gubernatorial candidate also vote for the candidate's party lower on the ballot, so those candidates "ride the coattails." The effect can also run in reverse, with an unpopular leader dragging down others.

Evidence shows the effect has weakened over time. Ticket-splitting declined as voters have become more partisan, and more people vote the same party for president and Congress. In 1980 and 1984, presidential winners helped their party's congressional candidates, but results in later years show smaller gains. The presence of a presidential race increases turnout, which can help one party in down-ballot races, though a popular incumbent might offer more modest help.

The reverse is the midterm penalty, in which the president's party typically loses seats in the midterm election, when the president is not on the ballot. Candidates and strategists consider coattails when deciding endorsements and campaign appearances, and when a nominee might harm the ticket. Researchers distinguish the effect from incumbency advantage, in which individual candidates' advantages matter more. It can also influence a generic ballot result.