The coattail effect or down-ballot effect is the tendency for a popular political party leader to attract votes for other candidates of the same party in an election. For example, in the United States, the party of a victorious presidential candidate will often win many seats in Congress as well; these Members of Congress are voted into office "on the coattails" of the president.
This theory is prevalent at all levels of government. Popular statewide candidates for governor or senator can attract support for down ballot races of their party as well.
This is prevalent in the United Kingdom and Canada especially in a general election. People have a tendency to vote on the basis of a political party instead of the MP for their area.
This also refers to the phenomenon that members of the U.S. Senate or House of Representatives are more likely to be voted for on a year of the presidential election than a midterm.
The "coattail effect" is not usually caused by popular candidates convincing swing voters to cast their ballots for their party, although this is not unheard of. Rather, the effect often stems from popular candidates driving voter turnout among their own party base, people who are likely to vote for down-ballot party candidates anyway.
The "coattail effect" has also been used to derogatorily describe the effect of Group Representation Constituencies (GRCs) in Singapore, where candidates for Parliament run on a party slate of 3 to 6 candidates. This allows weak candidates to get elected "riding on the coattails" of strong candidates on their slate.
Riding the coattails can be used as a generic metaphor that refers to one who achieves some level of success or notability primarily through association with someone else.
Presidential coattails
Presidential coattails is the ability of a presidential candidate to bring out supporters who then vote for his party's candidates for other offices. In effect, the other candidates are said to ride on the presidential candidate's coattails.
In the United States
Before the introduction of the secret ballot in the late 19th century, voters cast their ballots by taking a ticket provided by a party worker and putting it in the ballot box. The party-column ballot listed all candidates of the party in a single column and allowed the voter to mark off the party box at the top, which encouraged straight-party voting and the coattails effect. Straight-party voting was the norm, and winners in presidential elections often had long coattails. They almost always began their term with majorities in the House and Senate.
In modern times voting machines have replaced the party-column ballot with the office-column ballot: candidates are grouped by office rather than party. Often there is no way to cast a party-line vote, and each office must be voted on separately. The proportion of voters choosing House and presidential candidates of different parties increased from 13 percent in 1952 to more than 40 percent in the elections of 1972, 1980, and 1988. Consequently, Presidential coattails have been virtually eliminated in most elections, and a number of Presidents, including Richard Nixon, Ronald Reagan, and George H. W. Bush, have begun their terms with one or both chambers of Congress controlled by the opposition party.
Presidents may suffer from a "reverse coattail" effect in which their party's candidates for the House or Senate get more votes than the presidents themselves. In 1976, for example, Jimmy Carter won the White House with 40,831,881 votes, but Democratic candidates for the House that year received 41,749,411 votes. In 1992, almost all Democrats elected to Congress won more votes in their congressional districts than the party's presidential candidate, Bill Clinton; that may have had to do with the presence of a strong third-party presidential candidate, Ross Perot.