Coattail effect

The coattail effect is the tendency for a well known and popular political party leader to draw votes for other candidates of the same party in an election.

For example, some would argue that many Republicans gained seats in Congress and lower offices following the 2004 elections due to the 'coattail effect' of George W. Bush. On the Democrat side, the 1948 upset win of Harry S. Truman resulted in the Democrats gaining seats in the Senate and retaking the House; though Truman was expected to lose badly he had tremendous personal popularity resulting in a significant coattail effect.