Incumbents Polling Below 50% Often Win Re-Election, Despite Conventional Wisdom

[Joanne98]

By NATE SILVER
Read enough polling commentary, and you’re sure to encounter arguments like this one:

And a staggeringly high number of Democratic incumbents are … sitting below that magic 50 percent number.

That number should send shivers down the spines of Democratic strategists. In 2008, when Democrats coasted to victory across the board, 32 House Republican incumbents were under the 50 percent mark in the last poll of the cycle, and 14 of them lost — a 44 percent mortality rate.

That was from The Hotline’s Josh Kraushaar. Here is Real Clear Politics’ Sean Trende making a version of the same claim.

When dealing with incumbents, it is much more important to look at the incumbents’ number than the challengers’ number, since undecideds usually break against the incumbent.

And here is Michael Barone of the Washington Examiner:

Another metric is daunting for Democrats. Polls in House races almost always show incumbents ahead of challengers, because incumbent members of Congress are usually much better known than their opponents. An incumbent running below 50 percent is considered potentially in trouble; an incumbent running behind a challenger is considered in deep doo-doo.

http://fivethirtyeight.blogs.nytimes.com/2010/10/11/incumbents-polling-below-50-often-win-re-election-despite-conventional-wisdom/

[Clio the Leo]

1. KICK!!!!!!!!!!!

READ THIS PEOPLE!!!!

[pruple]

2. K&R

[FBaggins]

3. The conventional wisdom isn't that they always lose...

...it's that a lead doesn't mean as much if it's an incumbent polling under 50. It's certainly a bigger deal in senate races than house races. Even the senate races in Nate's first chart where the incumbent won demonstrate the "undecideds tend to break in the challenger's favor" statement. Leads by nine... wins by three. Leads by 16, wins by 7. etc. In total, roughly half of the listed examples demonstrate the phenomenon.

There's far more to it than simple math. It's isn't a magic formula, it's rule of thumb that rests upon underlying assumptions. If, for instance, a well known incumbent is not running against a relative unknown... but instead against an opponent who has also won statewide races in the past, you wouldn't assume that a sub-50 number was as significant.

Even simpler, ANY candidate polling under 50% (assuming a two-way race) knows that a win is theoretically possible if she can get the undecideds to break in her favor. The question is what gets them to break and by what percentages. It's just that the challenger of an incumbent usually has more to work with. Voters know the incumbent's record and would already lead that way if they were inclined to support that record.

It's silly to boil the rule down to "any time an incumbent is under 50%" when there's clearly a massive difference between Reid leading 45-44 and Burr leading by 49-36. It's equally a given that Reid would be far more comfortable with a 51-49 lead in a decent poll than with a 47-44 lead.

Nate is also looking at polls from 30 days prior to the election, which seems fine since that's close to where we are now, but each of those races moves to a greater or lesser degree during those final weeks (I know we're hoping these current races do).