I do, however, believe the Court could eventually rule on this issue - but only in connection with another case or controversy and only if Trump were convicted and disqualified and the validity of the disqualification were later challenged after an attempt was made to enforce it.
For example, suppose Trump were to be convicted and disqualified from holding future federal office and then try to run for president again. He would have to meet numerous state-specific filing requirements to demonstrate he meets the constitutional requirements to run for president. In such case, numerous states would likely refuse to put him on the ballot because he'd been disqualified in the impeachment process. Trump could sue, and as part of that suit allege that the disqualification was invalid because the Senate did not have the Constitutional jurisdiction to disqualify him.
The argument would be a simple one - Congress' impeachment of power extends only to one class of person: sitting presidents. Anyone outside of that class of people is outside of that scope of power and Congress has no right to impeach, convict and disqualify them. Doing so has no more legal weight then trying to impeach a governor or a mayor or a Member of Congress or the First Lady. I don't agree with that argument, btw. I think the Constitution clearly gives the Senate power to try and convict a former president, particularly one who was impeached while still in office. But I also believe that if the impeached person is outside of the Senate's jurisdiction, the impeachment and disqualification can be ruled unconstitutional and invalid.
In such case, I believe the Supreme Court could rule on the constitutionality of the process that disqualified him.
The question would be narrowly limited to whether the Senate had jurisdiction to try, convict and disqualify a former president. That would not be any interference in the impeachment process because this would be a narrow constitutional question that the Court could pass on - whether the Constitution's grant of impeachment jurisdiction included jurisdiction over a former president. This wouldn't be a question of whether the Senate action was appropriate. It would be a question of whether they had the power to do it at all. If former presidents don't fall within the Senate's jurisdiction, the court could rule the Senate had no right to impose a penalty.
That's just one example of an instance in which the court could rule on this. But it would have to be further down the line and only after the Senate's action had some some real and direct impact. I don't see how it could be done prior to that.
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