Attempts to measure intense participation by blacks and college youth for Obama drive the polls’ disparity. Some issues: Does traditional polling overlook the fact that many younger folk have only cellphones, which are ordinarily not called? Doesn’t the definition of a "likely voter" undercount the youth vote? But what about the fact that younger voter turnout is never as high as predicted? On top of all this is a political correctness before pollsters, which is then itself corrected in the voting booth.
More important, keep in mind some political history, especially when you see a poll showing Obama above 50% nationally: no Democrat has won a majority of the popular vote since LBJ in 1964 and FDR in his triumphs. Before that there is quite a long stretch of history--back to Franklin Pierce. Dems have typically been a sectional party, not a national majority party, neither by popular vote nor by electoral vote. BHO is no LBJ or FDR.