Spooks
Poor old Wilson, all those rumours spread by some quite batty MI5 types, yet Dennis Healy, amongst others, had once been in the Communist Party!
In the 1930's plenty were, as young men, frightened by the rise of Fascism, before the extent of Stalin's terror became widely known, the CP was the only force really against this.
Labour at the time had a pacifist leader, the Tories in government, was largely for appeasment.
But Healy and others who would become Labour ministers, did not stay in the CP for that long, if anything, just prior to WW2, the Hitler/Stalin pact saw to that, they would mostly go on to be as commited to Western defence in the Cold War, as others.