First, I'd like to be clear that I'm simply stating my opinion (actually the phrase "gut feeling" more closely describes it). I believe McCain would pull "slightly" more from the GOP, but with the razor-thin margins of Presidential election lately, slightly more would be enough.
But a third party movement has failed with John Anderson and some others.
That's correct, and I should have more carefully specified that I was referring only to third-party candidacies which were originated, funded and directed by the same Establishment "Powers That Be" that control the Democrat and Republican parties. I was not referring to genuine populist, grass-roots movements like Anderson, Wallace, Dixiecrats etc.
Another distinction is that the circumstances and issues in question have to be such that the third-party would only be able to pull enough votes from either the Dem or Repub side to simply shift the victory, and thus the direction of public policy, into an outcome that a normal election would not, but which is designed to accomplish certain specific objectives of the insiders.
The circumstances which give rise to genuine grass-roots third-party movements, however, generally mean that the third-party threatens to shift the election in a direction inimical to the insider Establishment. Their usual strategy to counter them is to ridicule or demonize them using the MSM, but if the threat they pose becomes too great, more drastic measures are likely to be taken (e.g. Wallace).
McCain is and has been throughout his political career a creature and a tool of the PTB's. If they feel the timing is right, he would be a perfect, expendable third-party candidate to expedite a drastic shift of control of the American government and an acceleration of our spiral into the abyss of world-order socialism.
If the immigration issue is not handled quickly, both parties are in dangerous territory. The Democrats lost to Abraham Lincoln on the slavery issue and it created the Republican Party; and they suffered with black voting rights issues in the South, so they should be more afraid of this than Republicans, IMHO.