If John Kerry loses...
...then he can blame it on two gratuitous and unnecessary comments he made in the debates that has given the Bush campaign ammunition to attack Kerry. And more importantly they sucked up critical airtime which has muted Kerry's attempt to get out his message of change (nee Fresh Start).
First Kerry comes out with the "global test" mantra, which although he did state it was not meant to give the international community a veto over American security, it allowed the Bush team to portray it that way. The effect of this has lasted throughout the final month of the campaign, though its effect has lessened a bit recently.
Second, Kerry, in trying answer a question by CBS' Bob Schieffer in the third debate about whether being gay was a choice or not, inserted a reference to Mary Cheney, the Veep's openly gay daughter, and created an unnecessary kerfuffle. Kerry's operatives had to respond to the accusation by the Bush team that Kerry was exploiting Mary Cheney's lesbianism for political purposes. Regardless of whether the Mary Cheney reference was an appropriate point to make (see Andrew Sullivan's comments) it created a political smokescreen that took away from Kerry's impressive showing in the debates, again.
What this underscores is that Kerry has a habit of giving his opponents a way to attack him not because they came up with something good but because Kerry keeps putting his foot in his mouth (see also the "nuisance" comment).