Women voters have received special attention from the presidential candidates in recent elections primarily because of differences between women and men in their political preferences, a phenomenon commonly referred to as the gender gap. Statistically, a gender gap can be defined as the difference in the proportion of women and the proportion of men who support a particular politician, party, or policy position. In the 2008 election, Senator Barack Obama received 56 percent of women's votes compared with 49 percent of men's, resulting in a gender gap of seven percentage points.

A gender gap in voting has been evident in every general election for president since 1980. In each of the previous eight presidential elections, a greater proportion of women than men have voted for the Democratic candidate. In 2004, when Republican President George W. Bush was reelected, 51 percent of women compared with only 44 percent of men cast their votes for his Democratic opponent, Senator John Kerry. Bush received just 48 percent of women's votes but 55 percent of men's, resulting in a gender gap of seven percentage points – the same size as in 2008.

Prior to the 1980 election, it was widely believed that women and men took similar positions on most issues, had similar political preferences, and voted in much the same ways. In other words, the assumption before 1980 was that gender did not matter much in voting.