In a British Journal of Political Science article (abstract here) excerpted from their recently published book, Mandate Politics, Lawrence Grossback, David Peterson, and James Stimson answer this question as follows.
bq. To interpret an electoral outcome as a mandate, members of Congress must believe (a) that the electorate has issued a clear instruction, and (b) that the outcome conveys an implicit threat that voters will exact a penalty if they don’t get the changes that they want.
The meaning that members of Congress attach to an election outcome depends upon the post-election interpretations that appear in the media. Working from that premise, Grossback, Peterson, and Stimson examined every post-election New York Times, Washington Post, AP, and UPI story, 1960-1996, that contained discussion of whether the election outcome was a mandate, recording in each instance whether the story supported or opposed the victors’ claim that they had been given a mandate.
In most years, the media commentary in these stories ran heavily against the idea that the voters had conferred a mandate.
Three elections — those of 1964, 1980, and 1994 — stood out from the rest, with a clear media consensus each time that a mandate existed.
Why those three years in particular?
bq. “The key … lies in the combination of what election-night commentators expect to see and then what they actually see as the returns come in. In other words, what matters to constructing a story is what is surprising on election night.” Unexpected and decisive results provoke surprise and produce a sense that a mandate has been issued.
bq. To test this interpretation, Grossback, Peterson, and Stimson created a measure of how much more one-sided the election outcome is than the historical norm. The result? “The message … could not be clearer: media consensus emerges in the presence of unusual partisan sweeps and not otherwise.”
I’ll leave for a subsequent post Grossback, Peterson, and Stimson’s answer to the logical follow-up question: What are the policy consequences of mandates, on those relatively rare occasions when they do occur? For now, I’ll simply note the irony that the media, which concentrate so single-mindedly during campaigns on their personality and horse-race aspects, seem nonetheless to play such an influential role in constructing policy-based interpretations of their outcomes.