The “incumbent rule” is a rule of thumb used by pollsters that says incumbents rarely get a higher percentage in the election than they receive in polls, and that voters still undecided on the very last poll tend to “break” disproportionately for the challenger.
Michael Barone: “The assumption has been that voters know an incumbent, and any voter who is not for him will vote against him.”
Polling Report: “It seems that undecided voters are not literally undecided, not straddling the fence unable to make a choice – the traditional interpretation. An early decision to vote for the incumbent is easier because voters know incumbents best. It helps to think of undecided voters as undecided about the incumbent, as voters who question the incumbent’s performance in office. Most or all voters having trouble with this decision appear to end up deciding against the incumbent.”
Nonetheless, empirical data suggests the rule may be a myth. Nate Silver notes that it is “extremely common for an incumbent come back to win re-election while having less than 50 percent of the vote in early polls.” In addition, “there is no demonstrable tendency for challengers to pick up a larger share of the undecided vote than incumbents.”