It doesn't trigger the vote if you pay the coalition members, form a Weimar coalition, or don't raise coalition dissent at all prior
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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.