The debate is raging in Washington and across the country. Many reporters and conservative commentators are calling Bush's victory a mandate, while liberal groups and other observers are denying that it is anything of the sort (Media Matters usefully rounds up both sides of the debate in the process of arguing the liberal side). All this raises an important question from a political science perspective - what are mandates and how do they work?
The best work I've seen on this is a recent American Journal of Political Science article by Jim Stimson, David Peterson, and two other political scientists (236K PDF). They define a mandate as essentially a social construction - a collective interpretation of election results that carries an informational signal to nervous incumbents worried about re-election. In response, members of Congress deviate from their normal voting patterns in the direction of the mandate for some period of time, particularly those whose winning margins decreased in the previous election (they give this period a half-life of approximately 150 days). The authors provide some useful empirical tests of this hypothesis, examining the 1964, 1980 and 1994 elections as the three most recent "mandate" elections (based on coding of media content). 1980 appears to have had by far the biggest impact on individual Congressional voting behavior.
So how does 2004 compare to those three elections? In 1964, Lyndon Johnson won 61% of the vote (compared to Barry Goldwater's 38%) and 486 electoral votes. In 1980, Ronald Reagan received 51% of the vote (compared to Jimmy Carter's 41%) and 489 electoral votes. And in 1994, Republicans received 52% of votes for the House (compared with Democrats' 45%) and picked up 52 seats, as well as eight seats in the Senate. By contrast, in 2004, Bush won with 51% of the popular vote (compared to Kerry's 48%) and 286 electoral votes, while his party picked up four seats in both the House and the Senate. Clearly, this election does not compare to the consensus "mandate" elections in terms of popular vote margins, electoral vote totals, or Congressional seat pickups.
However, given the closely divided nature of Congress from 1996-2002 and the 50/50 results of the 2000 presidential election, Bush's win is being portrayed as decisive. As mandate supporters point out, he's the first president to win 50% of the popular vote since his father in 1988, though that statistic is largely an artifact of Ross Perot's candidacy in 1992 and 1996. But in any case, perceptions are what matter here -- Bush will have a mandate if everyone thinks he has a mandate, especially those conservative Democrats who will determine whether he can pass his agenda in Congress.