By sending articles of impeachment to the Senate without having countered the president’s interference with House proceedings, Democratic leaders lost an opportunity. But there is a second shot at that confrontation, with the added benefit that any subpoenas the Senate issues will have bipartisan support. Calling a halt to Mr. Trump’s obstruction of Congress is essential if the process is to be impartial, independent and fully reflective of the conscience of our elected officials.

Second, it is critical to push back on the president’s dangerous expansion of executive privilege, regardless of whether the process results in his removal. Mr. Trump actually believes he is entitled to use his authority to prevent witnesses from testifying or to withhold incriminating documents or other evidence. Consider his open encouragement to witnesses to defy subpoenas, as well as the idea he floated that he would assert executive privilege to block current and former administration officials from testifying, for the sake of “protecting future presidents.” These moves are consistent with Mr. Trump’s view that Article II allows him to do whatever he wants.

If the Senate subpoenas witnesses, the Trump legal team will once more assert executive privilege, or the closely related idea of “testimonial immunity,” to block their enforcement, but this would be a gross abuse of that concept. The Senate must be prepared to go to the mat to enforce any subpoenas it issues, or the impeachment process itself will serve to weaken congressional authority and to establish the president’s entitlement to use the privilege of his office to immunize himself from scrutiny.

The enforcement of subpoenas in an impeachment trial is potentially complex. The Senate Impeachment Rules provide that “the Senate shall have power to compel the attendance of witnesses” and to “enforce obedience to its orders,” meaning that the Senate can use its inherent powers to enforce any subpoena it issues during an impeachment trial. But the sergeant-at-arms, who is empowered to conduct enforcement for Congress, does not have a jail or a full police force at his disposal. Moreover, Congress has not used its enforcement powers since 1934 (and the action prompted an immediate habeas petition from its recipient). This leaves open the possibility that the Senate will turn to the courts, not to rule on the validity of the subpoenas but to help enforce them. Alternatively, the witnesses themselves could turn to a federal court to defend against a Senate order to testify.

Either way, we can expect to see a pitched battle between Congress’ power and executive branch claims of executive authority play out in federal court. In such a scenario, there could be the unprecedented complication that the case could land in the Supreme Court while Chief Justice John Roberts is presiding over the trial. If Justice Roberts were to recuse himself, the country might face the specter of a Supreme Court deadlocked on the critical question of whether a sitting president undergoing an impeachment trial has the right to assert executive privilege to block witnesses at his own trial.