it's a very old historical thing with S and V. V was formed from S in 1917.
And in Sweden it was a truism that V shouldn't be allowed in goverment because they have a communist history. (Though they took away communism from the name in 1991).
Since V is the party most to the left, all right-wing parties want to keep it that way, thay V should never enter in goverment. And S also want to keep it that way.
Miljöpartiet doesn't have that history.
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