I know the strategy to stop McCain. The task is to figure out how to carry it out.
Sometime around last January or February, McCain when on the Letterman or Leno show to informally declare his candidacy for the presidency. While they were talking, the host (I forget which show--it was one of these two) said something about McCain's age. He intended it to be a joke. He didn't realize that it worried McCain.
McCain will turn 72 before the GOP convention. No president has been that age during the first year of their first term.
With the Congress, age doesn't appear to be as important with elections. In the Senate for example, Ted Kennedy will be turning 76; Robert Byrd is 90. There are probably some more in congress who are in that age range. However, with the presidency, people usually don't want to elect a president to a first term who is in the 70's.
In last night's FoxNews date, Chris Wallace mentioned at one point whether McCain would promise to, if elected, to not run for a 2nd term. McCain was pissed off that his age was mentioned, he knows it hurts him in this election. He tried to say how he doesn't want to make any declarations about it since he wouldn't want to be perceived by people as a "lame duck" during his first term.
So in conclusion, McCain's supporters need to be reminded about his age during the primaries, before it's too late and he ends up as the GOP nominee. How to do this, I'm not sure.