Do the undecided break strongly for the Challenger?

[Fmr. Gov. NickG]

Part of my undergrad thesis was actual about this exact phenomenon in congressional elections!

I did a study comparing polls in the last month or so of the election with the actual election results...using about 150 polls from the 1998 and 2000 election cycles.

What I found was that challengers outperformed the polls by an average of 3%, even when holding a number of other factors constant (including party, year, source of poll, etc).  The gains was statistically significant and normally distributed (I don't remember the SD off the top of my head...probably around 2%).

HOWEVER, there were a set of race in which the opposite was true.   These were races in which the challenger had what I call a "valance" advantage over the incumbent.  That is, where the challenger had better personal qualities, regardless of ideology.   Generally, these were race where:
 - The challenger was a former representive from that district or former statewide nominee
 - The challenger was a local celebrity
 - The incumbent was involved in a significant personal scandal

In these case, the incumbent actually outperformed his polls.  The incumbent's overperformance was similarly normally distributed around 3%.  
I think you might have seen this exact phenomenon at work in yesterday's Virginia primary involving Jim Moran.

Anyway, the rest of the thesis was about creating a rational-choice spatial model of voter preferences using these results, with the valence/ideology distinction at its core.

I don't know if this has any relevance to presidential elections, where both candidates are better known.

[Fmr. Gov. NickG]

Just to clarify a couple things about my thesis:

- I only looked at "competitive" congressional races.  (Defined by a certain margin of victory)  So these were almost all races where the challenger had chance to win and the national parties were pouring money in.  So the bump cannot be completely attributed to lack of recognition on the part of the challenger.

- I think I used polls from up to six weeks before the elections, but the 3% bump for the challenger held steady no matter when (within this time frame) the poll was taken...four weeks before the election or four days.  This is one of the variables I tested when testing counter-hypothoses.