A Kerry Presidency would do two very dangerous things...
1) It will place, in the White House, a party that would spend the next 4 years working hard to ensure their continued occupancy of that office. And, judging by the campaign this year, they're willing to do almost anything in that regard.
2) It will place, in the position of Commander-in-Chief, a person that in all likelihood will lose the war on terror thereby raising the spectre of a very different political landscape in 2008 and beyond where Libertarian "fiscal responsibility" will be very low on the totem pole of priorities.