I'm cutting and pasting the reference from Wikipedia's Layndon LaRouche entry since it addresses the issue most efficiently; he and hsi people are where the Obama/Hitler comparisons are coming from:
"LaRouche's comparisons of U.S. President Barack Obama to German dictator Adolf Hitler in 2009 have generated controversy. LaRouche has called Obama's actions "impeachable," without actually calling for impeachment, due to his support of health insurance reform that LaRouche says is comparable to Hitler's Action T4 euthanasia program.[156] The LaRouche movement has printed pamphlets with a picture on the front showing Obama and Hitler laughing together, and have made posters of Obama wearing a Hitler-style mustache.[157] In Seattle, police have been called twice in response to people who were offended by the posters threatening to tear them apart or to assault the LaRouche supporters holding them.[158] In one notable event, Rep Barney Frank referred to the posters as "vile, contemptible nonsense." [159]"
And again, Lyndon LaRouche is an activist and perennial presidential candidate, most recently and most frequently as a Democrat.