I found a good way to get KPD friendly——allow the mayday march to proceed without police presence.
but that would trigger a no confidence vote
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It's possible to get Weimar coalition from the May 1928 election if you focus on increasing popularity among the working class (and get lucky with card draws in the harder difficulties); increase in middle class support is mostly away from DDP and Zentrum, which is useless in trying to get to 50% total support for the Weimar parties.