Pew released an interesting poll yesterday, documenting the anatomy of the tightening presidential race. Like other recent national polls, the overall the survey shows a near dead heat (Obama leads 46 percent to 43 percent among registered voters with a +/- 2.5% margin of error). But looking behind the numbers reveals the basis of the McCain improvement in the past month. Pew believes two dynamics account for the Republican Senator’s enhanced fortunes in several national polls: