The irony is that because he was on the Senate Intelligence Committee he DID know they were communists for a fact,but could not produce the evidence without blowing counter-intel operations that were going on.
This goes back further than that,though. All the way back to the King Franklin Administration,when a small group of college professors were called before Congress to be questioned about subverting the minds of college students to make them into communist moles. There were at least 4 or 5 of them supeoned to testify before Congress,and guess where they stayed while refusing to testify? If you guessed "The Roosevelt White House",you are correct.
Every morning they were in DC they would get up in the morning,eat breakfast with the Roosevelts,and then ride in a WH limo with Eleanor to plead the 5th while laughing at Congress. They would do this over and over during the day,and then ride back to the WH at night for dinner and bed,and then get up the next morning and do it again.
This was no secret,either. There was a photo on the front age of the Washington Post of Eleanor sitting behind the defendants table and knitting while waiting for them to go back to the WH. When a reporter asked her why she was there,she told them "To show my solidarity with them". At one time I had a photo of that front page saved on my computer,but lost it when the computer died years ago. If you look on the web you can probably still find it,though.
BTW,for an added touch of irony,guess who was his senior council? Some guy from Mass named Bobby Kennedy. Odd how the left never jumps on the Kennedy Klan for "Red Baiting",ain't it?