I think they will oppose because if Yoon takes dictatorship, SK will become extremely shit and it will reflect very poorly on US.
It's not certain US wants SK to join in an eventual invasion of China. Imho it simply provides a front that's easy for the defenders to counterattack. Like it would have been if Russia brought Belarus into the SMO.
SK may be more valuable to America being in peace and trading with "both" sides, since they can help US replenish finished semiconductors, while China doesn't need the same help. Although, China would probably just block it regardless of SK neutrality.
The opposition party, which would stay neutral in an unprecedented US aggression against China, may be the best party for US interests.