For me, it was the ascendancy of Bill Clinton in the party I grew up supporting. I supported Democrats until the summer of '92. When I saw what was coming down the pike of the party I had joined, I walked away. For the first time I voted Republican.
Of course, I lost that election. And I lost in '96, too.
But 2000 and 2004 have been real nice. :)
My epiphany came when PBS of all stations broadcast a special with Milton Friedman, "Free to Choose." It was a life altering event. When I realized that America became what it did by virtue of the free market, not the government there was no turning back. Before this I voted for Carter, my first election. Afterwards, there was no turning back.