Despite what he wants you to think, Donald Trump has been Birtherism's biggest and most vocal champion for a long time now:
Trump himself may give a statement distancing himself from birtherism on Friday. But he shouldn’t be allowed to worm out of this so easily, because birtherism is in many ways the urtext of Trump’s presidential campaign. It demonstrates his willingness to mainstream fringe racism, his desire to flout the norms of political discourse, his ability to play the media, and his imperviousness to facts. And Trump has never truly been held to account for it during this campaign.
Yeah, we noticed that last part, Media.
You can read the whole history, and all of those classic Trump tweets (which he hasn't taken down as of publication, mind you) at Vox.