The race will be decided based on campaigning and advertising in the final two months, when voters finally start listening. The race will be decided based on the differences between our "likely voter" models and reality. The race will be decided based on economic performance in August and September. Mostly, the race will be decided based on the degree to which we can get every living conservative to the polls on election day, and the degree to which ACORN and SEIU can get every dead liberal to cast a ballot despite not having a pulse. We need to make this the highest turnout election ever, to make sure that we expand our margin in the House, take the Senate (likely but not assured), and have the mandate to force Romney to govern like candidate Romney, not like Governor Romney and the man he has been all his life.
Why the shifting make up of the sample space then? That number is well known at federal, state and local levels.