An advocate of coalition or a partner in a coalition(noun)
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Use "coalitionist" in a sentence
"Unfortunately the former pro-coalitionist Lib Dems have gone in the opposite direction, thereby becoming the most useless political party in the Senedd."
"Wish we could find another word than coalitionist."
"To-day the Bolsheviki were in power, while yesterday's coalitionist ministers and their co-workers found themselves cast aside and suddenly deprived of every bit of influence upon the further course of events."