How Many Voters Change Their Minds in the Month Preceding an Election?

This paper addresses a simple but basic question: How many voters change their mind during the month preceding an election? The question is simple and basic, yet the vast and rich literature on voting and elections has somehow managed to avoid it. Despite all the talk about the impact of campaigns, I have not come across estimates of how many people vote for a party other than the one they intended to support at the beginning of the campaign.