And I'll add to it, that Obama vs Romney is also only 1 battle in the war. If Obama wipes the floor with Romney tonight, many of Romney's donors likely shift all remaining money to competitive senate and house races and the Dems win the presidency but lose races down ticket.
Not that it was an intentional loss, but this could be a blessing in disguise. Romney's donors will still be plowing their money into his campaign now instead of down ticket races AND Obama is still in the lead. So, a poor Obama debate tonight may actually lead to a stronger democratic senate and house in November without changing the presidential election.