Most of the objections I've read involve his current nutjob activities. My objection has its basis in something from long ago:
Don't get me wrong. He's done and said plenty since then to disqualify him for any sort of Visa that would enable him to go anywhere in America beyond the UN. Until and unless we wise up and kick that gang of thieves out of Turtle Bay, there's not much we can do about him going there.
In my opinion, however, he should be arrested the minute he steps foot outside the UN building for acts committed in 1979.
And Columbia University should lose all Federal funding.
All of it, student aide, research grants, everything.
I don't think they have enough sense to be entrusted with my tax monies.