First, let’s dispense with Senate Republicans’ claim that impeaching a former officeholder is unconstitutional. It is far from clear that trials of ex-presidents are unconstitutional, particularly as the Senate has once before held an impeachment trial for an official after he left office. And while scholarship is mixed on the question, logic would seem to suggest that trials and convictions of former officials—especially officials who were impeached while they were still in office—are constitutional.
Second, Roberts is one of nine justices whose jurisdiction is confined to cases and controversies properly before the Court under Article III—he has no unilateral constitutional authority to interpret the Constitution’s language. Even if the question of Article I, Section 3’s meaning were put before full Court, it could not issue what are known as advisory opinions. Cases must emerge through the jurisdictional confines of Article III and its enabling legislation, lest courts start doing the business of the political branches.
Third, Roberts’s decision to absent himself from the Senate trial has stoked the flames of partisanship—not quelled them. Sen. Rand Paul (R-Ky.)—who moved unsuccessfully to dismiss the impeachment article as unconstitutionally too tardy for a trial—has argued that “If the chief justice doesn’t preside, I think it’s an illegitimate hearing and really goes to show that it’s not really constitutional to impeach someone who’s not president.”