Denouncing Patrick J. Buchanan as a ''Hitler lover,'' Donald J. Trump announced today that he was resigning his Republican registration in advance of a possible challenge to Mr. Buchanan in his expected quest for the Reform Party Presidential nomination.

''It's a very great possibility that I will run,'' said Mr. Trump, the real estate and casino millionaire. He timed his announcement for the eve of Mr. Buchanan's speech on Monday in which he is expected to quit the Republican Party and aim his Presidential campaign for the Reform nomination.

Mr. Buchanan brushed aside Mr. Trump's accusations of anti-Semitism as he courted Reform Party officials at a reception here today. ''To get in a slanging match with someone is not why I'm doing this,'' he told reporters outside the Westin Fairfax Hotel. ''We don't run negative campaigns. I'm not in this because I dislike other people.''

Mr. Trump is considered by Buchanan supporters as a straw man fronting for the wing of the Reform Party aligned with Gov. Jesse Ventura of Minnesota, a faction that is opposed to a Buchanan candidacy.