IMO, the slide got a real push start with LBJ and the Great Society. The Boomers couldn't vote in 64. Even a lot of my generation, the Silent Generation couldn't. But some of us could in 68 and more in 72 and we all saw how that turned out. The GOP brought on its own defeat in 76, but I'm sure more than a handfull of Boomers voted for Reagan and Bush in 80, 84 and 88. By 92, enough people were POd at both parties that a damaged 3rd party candidate still managed to garner more than 17% of the vote...and, because no one in power paid attention, 12% in 96. Now add all the recent minority immigrants (thanks to the Reagan amnesty) into the mix and we have the squeakers of 00 and 04 and with the way they have multiplied, we arrive at today. So to say it is ALL the Boomers' fault is far from accurate.
I have to agree; 1964 was also the year the Kennedys started pushing through their new immigration bill that gave preference to people from the third world and discouraged people from Europe.