I think the court is going to make its ruling based on the language of the 1977 statute in which the President can impose tariffs without approval of Congress in the event of a "national emergency."

Since "national emergency" is rather undefined, and apparently the statute in question gives the President the discretion to declare one, and further Congress has not spoken in opposition to Trump's tariff--and also, the Trump opponent's lawyer admitted that Trump has the power to do what he's doing no matter what, even if not under this particular statute, through other means;

I believe this Supreme Court will very grudgingly conclude that Trump does have the power to declare a national emergency and impose these tariffs; and that if Trump's actions constitute a misinterpretation of what Congress intended, Congress always has the ability to clarify its intent by amending or modifying the 1977 law.