At the time, he was believed to be the highest-ranking former Bush official to cross over; that designation now belongs to Paulson. Other former Bush officials and top GOP foreign policy minds of the era, including Max Boot and Robert Kagan, previously made the same call. Even more have said they won't back Trump but have stopped short of endorsing Clinton.
Here's the crux of Paulson's decision:
Republicans stand at a crossroads. With Donald Trump as the presumptive presidential nominee, we are witnessing a populist hijacking of one of the United States’ great political parties. The GOP, in putting Trump at the top of the ticket, is endorsing a brand of populism rooted in ignorance, prejudice, fear and isolationism. This troubles me deeply as a Republican, but it troubles me even more as an American. Enough is enough. It’s time to put country before party and say it together: Never Trump.
Paulson moves on to eviscerate Trump's business acumen, in detail, and then questions whether he has what it takes to actually be president. He doesn't dwell on why he'll back Clinton, except to say that he wants to do whatever it takes to stop Trump.
When it comes to the presidency, I will not vote for Donald Trump. I will not cast a write-in vote. I’ll be voting for Hillary Clinton, with the hope that she can bring Americans together to do the things necessary to strengthen our economy, our environment and our place in the world. To my Republican friends: I know I’m not alone.
The lack of support from Bush world, of course, extends to the family itself. The two former presidents Bush have declined to endorse Trump, and Jeb Bush, who clashed repeatedly with Trump in the Republican primary, has said he won't vote for either Trump or Clinton.