Incumbents and that 50 Percent Poll Rule

by: Hopeful

Thu Feb 25, 2010 at 06:26:53 PM EST

I wanted to bring your attention to an interesting post by Nate Silver of 538.com on The Myth of the Incumbent 50% Rule. He's prompted by the claim that incumbents get only the same percentage of the vote as they do in early polling, and therefore incumbents under 50% almost always lose. Here's his conclusion:

1) 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.
2) In comparison to early polls, there is no demonstrable tendency for challengers to pick up a larger share of the undecided vote than incumbents.
3) Incumbents almost always get a larger share of the actual vote than they do in early polls (as do challengers). They do not "get what they get in the tracking"; they almost always get more...
It is probably OK to focus on an incumbent's vote share in early polls while downplaying the challenger's number, but if you do, you need to add 6-7 percent to it to have the most accurate prediction of his likely performance in November.

Silver's data, which you can see by clicking through, shows that incumbents who poll 45% in January-June usually win -- like Frank Lautenberg in 2008, at only 45.7%, who got to 56% on election day -- and overall 19 of 30 incumbents under 50% end up winning. (Incumbents over 50% are safe.) In short, undecideds do not usually break overwhelmingly to the challengers as conventional wisdom tell us. Mark Blumenthal of Pollster.com says Nate is right, and explains that the "myth" is largely based on old races. I recall, but can't find the link, that kos recently found half of incumbents polling under 50% lose and half win. Many Democratic learned this lesson in the 2004 Presidential election, where Bush won despite many polls putting him under 50%. Here in New Jersey, Menendez won despite being below 40% (though he was appointed) and Corzine lost, but did get that 7 point surge.

I can't help but wonder if this speaks to the famous New Jersey election effect -- that Democrats poll too low. Perhaps instead of looking for a Jersey explanation, it's just that Democrats here poll the way incumbents do nationwide.

Hopeful :: Incumbents and that 50 Percent Poll Rule

Tags: 50 percent rule, incumbents, polls, Nate Silver, Frank Lautenberg, Bob Menendez, featured, (All Tags)

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Incumbents and that 50 Percent Poll Rule | 1 comments

I was taught (4.00 / 1)

that, unless there is compelling reason to believe otherwise, the "undecided" part of the early polls will tend to vote pretty closely to the way that those who are decided will vote. In other words, if you have someone polling at 40% and someone at 35%, with 25% undecided, you can expect 40% or so will go with the leader...perhaps a bit more if it's an incumbent.

Corzine's problem, as I recall, was that he polled in the middle-30s to lower-40s pretty much for a year prior to the election. It wasn't so much that he under-polled at all.

by: Thurman Hart @ Fri Feb 26, 2010 at 00:39:26 AM UTC