In his own words, Alinsky fought commies

Your only hope is to move first, to beat the Communists at their own game, to show the people you're more interested in their living conditions than the contents of your collection plate. And not only will you get them back again by supporting their struggle, but when they win they'll be more prosperous and your donations will go up and the welfare of the [Catholic] Church will be enhanced. ~ Saul Alinsky [1]

This is not to say that he did not admit to having any involvement in with the communists.

I knew plenty of Communists in those days, and I worked with them on a number of projects. Back in the Thirties, the Communists did a hell of a lot of good work; they were in the vanguard of the labor movement and they played an important role in aiding blacks and Okies and Southern sharecroppers. Anybody who tells you he was active in progressive causes in those days and never worked with the Reds is a goddamn liar. Their platform stood for all the right things, and unlike many liberals, they were willing to put their bodies on the line. Without the Communists, for example, I doubt the C.I.O. could have won all the battles it did. I was also sympathetic to Russia in those days, not because I admired Stalin or the Soviet system but because it seemed to be the only country willing to stand up to Hitler. I was in charge of a big part of fund raising for the International Brigade and in that capacity I worked in close alliance with the Communist Party... But all in all, and despite my own fights with them, I think the Communists of the Thirties deserve a lot of credit for the struggles they led or participated in. Today the party is just a shadow of the past, but in the Depression it was a positive force for social change. A lot of its leaders and organizers were jerks, of course, but objectively the party in those days was on the right side and did considerable good.[1]

Back in the 1930's, if you were opposed to racism, sometimes the Communist Party was the best choice of few options.   Frank Oppenheimer joined to protest the draining of a pool after African Americans swam there for one day.  African Americans were thought to be more germ ridden.

Well, he was - he did join the Communist Party - which Robert did not - in the 1930s, and, you know, a lot of this research was new to me. There were living in Pasadena. He was at Caltech, where he was getting a PhD candidate, and his brother was there, as well. And it turned out that the Communist Party were the only people who were, for example, protesting the fact that the Pasadena public pool was drained - I think it was Thursday - after the blacks were allowed to swim there for their one day. And he was outraged that no one else was addressing racism. And, of course, the Spanish Civil War was the other thing that he thought nobody was addressing.

So he and his new wife, Jackie, who was part of the - she was more associated than he was. They joined the party. And, of course, they immediately thought it was ridiculous because for one thing, they asked him to choose an alias, which he just thought was dumb. So he chose Folsom - which is, of course, the prison, Folsom Prison - as a joke. And in all the FBI files, they have him as alias Frank Folsom. [2]

As with many people, it came back to bite him.

And the HUAC hearings and - you know, to make a long story short. As you know from the book, it's a little bit complicated how exactly he got in trouble. But the interesting thing was he did get fired. He did admit he was a communist. He did not testify against anybody else, which was something that other people didn't do. They actually didn't name names, but did not plead the 5th, either. He and Jackie both - his wife Jackie both talked completely openly about their experience, but refused to talk - so they were in contempt of court. Actually, Clifford Durr, which I didn't know was the only attorney who would take them on. [2]