I'm not sure how I should read this. Would you break it down for me, or tell me why come is in its original form?

The communists I'd known in the past had always operated on the assumption that come the revolution, they'd be the ones lying around party headquarters with clipboards in their hands.

Thank you,

mitsuwao23come the revolution
This is an idiom with "come". "come" is invariable.

come the revolution = when the revolution came

This is not a commonly used idiom, but here are a few more examples.

Come spring, I'm moving to Florida. (When spring comes, ...)

Come the Rapture, the internet will be down. (When the Rapture comes, ...)
The Jones family planned to leave come the morning. (... when morning came. / ... in the morning.)

Thank you for the reply.

I looked it up for more details but couldn't find the idiom. Does that always come with "the revolution"? Or are there any other ways to use this "come"?

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I revised my previous post to give you some examples.
Thank you! PerfectEmotion: smile