From Ballotpedia:
Donald Trump donated $175,860 more to Democrats than Republicans from 1989 to 2010, but in 2011 his giving to Democrats significantly decreased. From 2011 to 2015, Republicans received $630,150 in donations from Trump, while Democrats received just $8,500. What caused Trump’s decision to abandon the Democratic Party in 2011 and throw his full support behind Republicans is unclear. It could have been his desire to be taken seriously in the political world and gain influence within the Republican Party in preparation for a future presidential run, as New York Times reporters Maggie Haberman and Alexander Burns suggested in their outline of Trump’s effort to court members of the conservative establishment beginning in 2011.