I dont think that matters, although I take your point about the time left for them to "fight" McCain but lets face it they do not need much time.
The amount of publicicty and air time given to both Obama and Clinton lately means that the American public is already warmed up to them. Once the winner of the Democratic Nomination is decided they will wipe the floor with McCain.
McCain, despite being popular with the conservative centre ground, is still viewed as a hard nosed Republican with some right of centre tendancies. It is my opinion that the USA wants to take a step to the left after having Old Warmonger Dubya in power for so long. Americans will vote Democrat and either Obama or Clinton will be next President, that is why they are fighting so hard for this nomination in the Primarys.