Coattail effect

From Conservapedia

The coattail effect is the tendency for a well known political party leader to draw votes for other candidates of the same party in an election. A leader of one political party who is receiving a vast amount of votes is often able to help attract votes for other candidates in the same party.

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.