The ruling party will have to put its own house ...... order.

The ruling party will have to put its own house in order.
In this context, the phrase "put its own house in order" means to resolve internal issues or conflicts within the ruling party and establish proper organization and discipline. The preposition "in" is used to indicate the location or state of being, conveying the idea of arranging or organizing the party's affairs internally. Therefore, "in" is the most suitable choice to complete the sentence.