Andrew Gelman and Gary King. "Party Competition and Media Messages in U.S. Presidential Elections," The Parties Respond: Changes in the American Party System, L. Sandy Maisel, ed., 2nd edition, Boulder, Colorado: Westview Press, 1994, Pp. 255-295 copy at http://gking.harvard.edu/files/abs/partycomp-abs.shtml. (Article: PDF).

Abstract

At one point during the 1988 campaign, Michael Dukakis was ahead in the public opinion polls by 17 percentage points, but he eventually lost the election by 8 percent. Walter Mondale was ahead in the polls by 4 percent during the 1984 campaign but lost the election in a landslide. During June and July of 1992, Clinton, Bush, and Perot each had turns in the public opinion poll lead. What explains all this poll variation? Why do so many citizens change their minds so quickly about presidential choices?

Also see related research.