Donald Trump is a Democrat.
He has given vastly more money to Democrat causes. He supported late term and partial birth abortion until a conversion of convenience to secure Republican votes.
He was for open borders until that issue worked against him in Republican primaries.
He is friends with Hillary and Bill Clinton, who he called before getting into the Republican primary.
He opposed tea party, conservative candidates in 2010 and 2014.
A vote for Hillary Clinton or Donald Trump is a vote for Democrats in November.
I will vote for a third party candidate if Donald Trump is the Republican nominee. Because if he is, there is no meaningful difference between the Democrats and Republicans.