It's debatable how much of a Democrat he really was. He did donate and befriend many of them prior to running.

After looking up it up, I didn't really know he registered as a Republican in 1987 and then changed it again (several times).

Trump registered as a Republican in Manhattan in 1987 and since that time has changed his party affiliation five times. In 1999, Trump changed his party affiliation to the Independence Party of New York. In August 2001, Trump changed his party affiliation to Democratic. In September 2009, Trump changed his party affiliation back to the Republican Party. In December 2011, Trump changed to "no party affiliation" (independent). In April 2012, Trump again returned to the Republican Party.[24]

In a 2004 interview, Trump told CNN's Wolf Blitzer: "In many cases, I probably identify more as Democrat," explaining: "It just seems that the economy does better under the Democrats than the Republicans. Now, it shouldn't be that way. But if you go back, I mean it just seems that the economy does better under the Democrats [...] But certainly we had some very good economies under Democrats, as well as Republicans. But we've had some pretty bad disaster under the Republicans."[25] In a July 2015 interview, Trump said that he has a broad range of political positions and that "I identify with some things as a Democrat."[24]