First, forget the Gallup Poll. Last week it had Kerry leading. Opinion does not shift 10 points in one week.
What matters is Bush's final poll number, not Kerry's. This LA Times article explains why:
http://www.latimes.com/news/politics/2004/la-na-fifty18oct18,1,6516901.story?coll=la-home-headlinesPolls Put Bush on the Edge
The president leads most surveys, but hovering at 50% leaves him little room for error.By Ronald Brownstein, Times Staff Writer
WASHINGTON — While most of America is watching the spread in the polls between President Bush and Sen. John F. Kerry, key strategists in both parties have their eyes on a different set of numbers: Bush's share of the vote and his job approval in the final surveys before election day.
Analysts watch the incumbent's numbers in the polls so closely because most voters who stay undecided until the very end of a presidential campaign traditionally break for the challenger. As a result,
challengers often run ahead of their final poll results, while incumbents rarely exceed their last poll numbers. "We know from the history of presidential elections that when a president is polling below 50% going into the election, he usually loses," said Alan I. Abramowitz, an Emory University political scientist. "That is true of incumbent office holders in general. The incumbent usually ends up getting the percentage that he is getting in the final polls — that's it."