Depends on what do you mean by the ‘left’ bolsheviks often viewed social-democratic and (to a lesser extent) liberal parties as their greatest threats. They were all appealing to the same demographics but were diametrically opposed on certain core points (democracy, free speech, political oppression etc.) making any real cooperation impossible.

The Weimar republic was a great example of that, the liberal/socialist government which was in power during it’s entire lifetime was so unstable and eventually collapsed because a significant proportion of the electorate continued voting for the bolsheviks. They wouldn’t even cooperate to prevent the far-right from coming to power (of course it’s not like they had a choice, moderate voters would have immediately shifted to the right if the socialists tried working with communists).

So it’s not that surprising. Stalin had no issue working with Hitler and supplying Germany with raw material which were necessary for their conquest of France (during the period western communist parties suddenly became rabidly pro-nazi..) until all of that blew up in his face…

Of course that was already quite different in the 60s and 70s after the Soviet were no longer as focused on ideological purity (and mass murder by extension).