Media Created 'Red' States, 'Blue' States
The division of the U.S. into "red" states and "blue" states is entirely the product of American media outlets, with the current color assignment dating back only to the 2000 election.
The delineation is commonplace now, as evidence most recently by President Obama winning 25 "blue" states (and the "blue" District of Columbia) and Mitt Romney winning 25 "red" states.
The convention of referring to Republican-leaning states as "red" and Democratic-leaning states as "blue" is a reverse of a longstanding tradition, maintained in other countries, of referring to political parties at the "left" end of the spectrum, meaning more liberal, as red and to "right"-leaning parties, more conservative, as blue.
American historical records as far as back as the 1888 presidential election show states won by the Democratic candidate that year, Grover Cleveland, as "red" on a color map and states won by that year's Republican candidate, Benjamin Harrison, as "blue".
The colors haven't always been red and blue. A 1908 pre-election edition of the New York Times colored states won in the previous election by then-President Theodore Roosevelt, a Republican, as "yellow" and states won by the Democratic candidate, Alton Parker, as "blue." In the same year, the Washington Post colored Republican states "red", Democratic states "blue", and undecided states "yellow".
Some research suggests that early television broadcasts of election night returns colored states won by the incumbent as "red" and states won by the challenger as "blue". Other sources say that the major TV networks varied their colors depending on network preference. For example, in 1976, NBC colored Democratic states "red" and Republican states "blue," whereas ABC's colors that year were "blue" and "yellow." Newspapers generally picked their own colors as well.
Because the final results of the 2000 presidential election were on hold for so long, the nation's major media decided to stick with the version that has Republicans "red" and Democrats "blue." (Some commentators refer to so-called "swing states," which are not immediately identifiable as strongly in the camp of one political party or the other, as "purple.")