The comparisons currently being made on the American right between Barack Obama and Adolf Hitler are at once absurd and obnoxious. They're absurd because Obama isn't remotely like Hitler, whatever the putative similarities between the two of them being confected. And they're obnoxious because the point of purveying the analogy is to let the idea of a likeness spread, so to speak, from the particular similarity proposed to the whole of the Obama persona, so that he becomes the very incarnation of evil in progress, evil on the march. This is absurd and obnoxious in the same way that the Israel/Nazism analogy is. It is absurd and obnoxious in the same way that the Bush=Hitler stuff was.
All that said, I don't think it's automatically true to say that 'likening... Obama to Hitler is the functional equivalent of calling for an act of violence against the president of the United States'. The comparison is indeed poisonous. It creates a terrible atmosphere in public debate. And to the extent that there's a real danger of someone, somewhere, having a pot at Barack Obama, it increases that danger. But no one who proposes a comparison between two persons is thereby committed to the view that they are alike in all respects. Therefore 'functional equivalent' is too strong. There may be some - loonies as may be - who think that Obama's health policies bear comparison with those of the Nazis, but for whom saying so doesn't amount to calling for violence against the president.
Those liberals and leftists who either indulged in the Bush-Hitler talk or, refraining from it for their own part, took an indulgent view towards it amongst others, might now care to reconsider its legacy for democratic political debate in the US.