I don't think anything could have convinced the left any more than it already knew. Many of the leaders of the left were already involved in espionage. Those that weren't, for the most part, thought Soviet espionage was a good idea. Moynihan was a good man, but he had a naively rosy view of the intentions of his fellow travelers on the left.
I respected Moynihan.
Even more so when I learned he was not fooled by Hillary.