Since 1932, Contra Costa County has been a Democratic stronghold in presidential elections. It temporarily leaned toward the Republican Party in the 1970s and 1980s, with successive presidential wins by Richard Nixon in 1972, Gerald Ford in 1976, and Ronald Reagan in 1980 and 1984. Reagan was the last Republican presidential candidate to win the county.

Democrats hold wide advantages in voter registration numbers in all political subdivisions in Contra Costa County. The Democrats' largest registration advantage in Contra Costa is in the cities of Richmond, where there is a 60.3% registration advantage with 3,192 Republicans (6.2%) out of 51,395 registered voters compared to 34,159 Democrats (66.5%) and 12,095 voters who have no party preference (23.5%); El Cerrito, where there is a 59.0% registration advantage with 1,169 Republicans (7.4%) out of 15,877 registered voters compared to 10,543 Democrats (67.6%) and 3,654 voters who have no party preference (23.0%); and San Pablo, where there is a 58.3% registration advantage with 641 Republicans (6.1%) out of 10,550 registered voters compared to 6,793 Democrats (64.4%) and 2,746 voters who have no party preference (26.0%).