Elections are influenced by many variables. Some political scientists speculate there is a "coattail effect" (when a popular president or party position has the effect of reelecting incumbents who win by "riding on the president's coattails"), although there is some evidence that the coattail effect is irregular and possibly declining since the 1950s. Some districts are so heavily Democratic or Republican that they are called a safe seat; any candidate winning the primary will almost always be elected, and these candidates do not need to spend money on advertising. But some races can be competitive when there is no incumbent. If a seat becomes vacant in an open district, then both parties may spend heavily on advertising in these races; in California in 1992, only four of twenty races for House seats were considered highly competitive.