I suppose in theory the “federal government” might be a party in a lawsuit as a legal person, but it is never done that way. There is no fictitious entity called the “Federal Government” or the “U.S. Government” or whatever. One of the reasons would be that we have a divided government - no single authority can claim to speak for the whole government.
Consequently, the federal government participates in lawsuits in four principal ways:
- In criminal proceedings initiated by an indictment obtained by U.S. attorney as the “United States.”
- In civil proceedings under the name of the government agency involved, e.g., “U.S. Department of Health and Human Services”
- In civil proceedings under the name of the chief administrator of the government agency involved, e.g., “Jonathan W. Dudas, as director of the U.S. Patent and Trademark Office.”
- In cases that are brought by an independent prosecutor (which are created on an ad hoc basis), under whatever name has been assigned to that office, either under the name of the independent prosecutor (“Kenneth Starr”) or some other name (“the Office of the Independent Prosecutor”).
Again, in the U.S. system, there is no entity that can be called the “Government.”
The U.S. solicitor general is a subordinate of the U.S. attorney general, as are all the 90-plus U.S. attorneys. The U.S. attorney general is the chief legal advisor to the U.S. president. The U.S. solicitor general’s sole role is to represent the president in oral arguments before the U.S. Supreme Court. (In British terms, you might say that the U.S. solicitor general is like a barrister who is instructed by the U.S. attorney general, who is like the president’s chief solicitor.)
So, no, the solicitor general would not be charged with advising the president in a case like the one you propose.
As I said in an earlier post, it would be very rare for the federal government to need specific advice on state law. Generally speaking a state’s own state attorney general would be responsible for such expertise. In the rare case that this might happen, the U.S. attorney general would still be the president’s chief legal advisor. I’m not too familiar with the inner workings of the U.S. Department of Justice (the agency headed by the U.S. attorney general), but I WAG that the attorney general would probably assign or commission the specific kind of expertise needed on an ad hoc basis.
Matters implicating only a portion of the executive branch, including the Executive Office of the President (the “White House”) itself, would seek advice from their own in-house attorney staffs in cooperation with counsel supplied by the U.S. Department of Justice.
Note that there are entitles of the federal government that do not fall under the authority of the U.S. president and thus would not be advised by the U.S. attorney general or his department. In particular, members of the U.S. Congress, if they were to become parties to a legal action in their official roles, would have to obtain their own legal counsel. The Congress does appropriate funds for the maintenance of extensive staffing of legal advisors. Each legislative committee has a committee counsel, for example.