Replying to God - The reason I made that prediction is really quite simple. I'll list the reasons one at a time:
1) Consider that Obama and Clinton are already diverting most of their time and resources to each other, mostly ignoring McCain in favor of defeating their respective rival. This creates a large blind spot to the Republicans, who are able to act unimpeded by the same degree of internal conflict.
2) Consider that the research, strategizing, funding, planning, etc are more directed towards their Democratic counterpart then McCain, and that this represents time and resources that they can never get back. McCain can gather resources, strategize, and strike without a rival holding him back.
3) Both sides are fighting dirty now. Both have gotten into the proverbial mud and engaging in heavy mudslinging, basically making both look like pigs. McCain has mostly stayed behind the scenes and has a clean record by politician standards. All he has to do is play up on that and appear like the political angel to the public.
4) Regardless of who wins in the Democratic party, that candidate will probably be exhausted by the time they face McCain. Even if the candidate is very strong willed or has excellent emotional management skills, many months of fierce rivalry, constant worries, neverending battle and political maneuvering is going to take a heavy toll on both sides. Whoever faces McCain will mostly likely be under a degree of battle fatique while the Republicians charge in with a fresh candidate.