McCain was leading in the polls until the infamous Palin interview with Katie Couric.
Not really, Obama had a fairly clear and consistent lead for the vast majority of the campaign. There was a brief burst of positive polling for McCain around the beginning of September, but Obama was already winning again by the time the Couric interview aired. (September 24th)
http://www.realclearpolitics.com/epolls/2008/president/us/general_election_mccain_vs_obama-225.html
