The following from the August 23rd New Yorker may be at least one reason for Bush's latest decision to focus more on his base (and less on swing voters):

Boyd’s assessment is echoed by Tony Fabrizio, of the Republican polling firm Fabrizio, McLaughlin & Associates, who issued a gloomy memorandum last month on Bush’s prospects among swing voters. “Our analysis of ‘undecided’ voters in our most recent Battleground State Survey reveals that they are currently poised to break away from President Bush and to John Kerry based on the following findings,” he wrote. “They are more than twice as likely to see things headed down the wrong track as compared to voters overall. They give President Bush a net negative image rating. They give President Bush a net negative job-approval rating. A solid majority sees the country as being worse off than they were four years ago. They are significantly more pessimistic about the current state of the nation’s economy. They are significantly more pessimistic about their own current financial condition. They are twice as likely to see the number of jobs in their area as decreasing instead of increasing. They are significantly more likely to favor the federal government doing more as opposed to doing less. They are more likely to be pro-choice on the issue of abortion. They are more likely to have seen or heard advertising critical of President Bush than of John Kerry in the past year. John Kerry holds a slight net positive image rating.” In conclusion, Fabrizio wrote, “Clearly if these undecided voters were leaning any harder against the door of the Kerry camp, they would crash right through it . . . which would hand Kerry a lion’s share of these states.”
Virtually all the indicators that Fabrizio catalogued tipped further in Kerry’s favor after the Democratic Convention in Boston.

And note this assessment is from a Republican polling firm!