Why does the McCain campaign stretch the truth, when Obama has so many legitimate problems?

In any other election year, Obama would not have made it past the first month or two of the primaries with ties to domestic terrorists, loan scandals, racists, and laundering earmarks into his own coffers. Yet this year, with Republican popularity at a low, he is the Democratic nominee. Why does McCain stretch the truth on his advertisements when there are so many legitimate complaints he could be focusing on?