Peter Kotz argues, wrongly I think, that the issue with Keith Ellison is that he is a Muslim per se. Even then, Kotz acknowledges that Ellison has a radical Islamic past: http://www.citypages.com/news/mild-mannered-keith-ellison-is-about-to-become-americas-scariest-muslim/401316426
My opinion, at least, is that the Democratic Party - like so many Progressive entities in the West - has been co-opted by PC Identitarianism. Their idea "diversity" is entirely flawed. They cannot help themselves: https://quadrant.org.au/opinion/qed/2016/11/gloriously-unhinged-president-trump/
Everybody who belongs to their Rainbow of Discontents (Ellison et al) gets a free pass; everybody who opposes them (Steven Bannon et al) is an anti-Semite, racist, fascist, white supremacist, mysoginist, xenophobe.
Here, by the way, is a critique of Ellison very different from Kotz's. It was written in 2010 and comes from the Middle East Forum: http://www.meforum.org/2756/keith-ellison-stealth-jihad
This might be interesting, too: http://freebeacon.com/politics/jewish-leaders-question-chuck-schumers-endorsement-of-keith-ellison/