[Lurking Libertarian]

In many cases, the U.S. Government, or one of its agencies, is a party to the case, so the DOJ argues the case. In cases where the Government is not a party, the DOJ may, but ordinarily doesn't have to, file an amicus curiae ("friend of the Court") brief, setting forth its views on how the case should be decided. Occasionally, SCOTUS itself will "invite" the DOJ to file an amicus brief; the "invitation" is considered tantamount to an order-- it has never been declined.

As others have pointed out, the argument for the Government is usually made by the Solicitor General or his staff, but the Solicitor General's office is part of the DOJ. Occasionally, the Attorney General himself will argue a case instead of the Solicitor General.

[NoLibZone]

What if the WH doesn’t agree with the law being heard?

Do they still have to file an Amicus Brief?

Or can they stay away?