Answer:
The term is bipartisanship.
Step-by-step explanation:
If the two parties agree on promoting a piece of legislation together that enjoys wide support within each of them, we speak of a "bipartisan bill."
Or if Congress declares war on a foreign country unanimously, then we say that the declarion of war was an act of bipartisanship because all Congress members, no matter which party they belong to, voted in favor.