Trump has been a Democrat and Independent before running as President (see Flip-Flops).
Trump's donations to Democrats since 1990 election cycle included donations to:
He has called himself Obama's biggest cheerleader, and he invited Hillary & Bill Clinton to his wedding. Trump was generous in his praise of Hillary before he began running as a Republican candidate. Trump also donated between $100,000 to $250,000 to the Clinton Foundation.
(1) Amy Woods, "Trump: I Cheered for Obama, but He's Dangerous, Oct. 19, 2011, Newsmax.com, http://www.newsmax.com/InsideCover/Trump-Obama-class-warfare/2011/10/19/id/414955/
(2) Chuck Ross, "Hillary Clinton HAD a Front-Row Seat at Donald Trump's Most Recent Wedding," Daily Caller, June 16, 2005, http://dailycaller.com/2015/06/16/hillary-clinton-had-a-front-row-seat-at-donald-trumps-most-recent-wedding/
(3) Zeke Miller, "When Donald Trump Praised Hillary Clinton," Time, July 17, 2015, http://time.com/3962799/donald-trump-hillary-clinton/
(4) Danielle Kurtzleben, "Most of Donald Trump's Political Money Went To Democrats - Until 5 Years Ago," NPR, July 28, 2015, http://www.npr.org/sections/itsallpolitics/2015/07/28/426888268/donald-trumps-flipping-political-donations
(5) Zachery Newkirk, "Donald Trump's Donations to Democrats, Club for Growth's Busy Day and More in Capital Eye Opener: February 17, OpenSecrets.org, February 17, 2011, http://www.opensecrets.org/news/2011/02/donald-trumps-donations-to-democrats/
(6) Alex Griswold, "Donald Trump Donated At Least $100K to Clinton Foundation," DailyCaller.com, May 14, 2015, http://dailycaller.com/2015/05/14/donald-trump-donated-at-least-100k-to-clinton-foundation/