President Trump’s Republican allies want the Senate to hold an impeachment trial to prove that the president is not guilty of high crimes and misdemeanors. But Democrats in the House are standing in the way.

While the House approved articles of impeachment last December, its Democratic majority has not yet appointed the members who will take those articles to the Senate and manage its case in the subsequent trial there. Until the House takes these steps, the Senate can neither convict Trump nor acquit him, because its rules stipulate that the House must officially inform senators of its decision to impeach the president before a trial can begin.

According to House Speaker Nancy Pelosi of California, the House will not complete its share of the impeachment process until Senate Majority Leader Mitch McConnell, a Republican from Kentucky, and Minority Leader Chuck Schumer, a Democrat from New York, agree on the ground rules that will govern a Senate trial. Amid this impasse, support among Republicans to hold a trial without waiting on the House has gained traction. The top Republican on the Senate Judiciary Committee, Lindsey Graham of South Carolina, is now calling on his colleagues to change the rules to authorize a Senate-only trial.

Proponents of unilateral Senate action argue that it is permitted because the Constitution supersedes any internal rules prohibiting senators from holding a trial without the House. Some proponents even assert that the Constitution compels the Senate to act unilaterally. The Wall Street Journal contends that the Senate “has a responsibility to fulfill its part of the Constitution’s impeachment duty as a check on the partisan excesses of the Pelosi House.”

Notwithstanding the partisan excesses of either chamber, senators risk violating more than their own rules if they heed such advice. By choosing to hold an impeachment trial without the House, senators are circumventing Congress’s bicameral structure and undermining the Constitution.

Article I, section 2, clause 5 of the Constitution gives the House the “sole Power of Impeachment.” Article I, section 3, clause 6, gives the Senate the “sole Power to try all Impeachments.” The Constitution, therefore, empowers the Senate to hold a trial to convict (or acquit) a government official only after he or she has been impeached by the House.

This raises the question of when the Constitution authorizes the Senate to conduct an impeachment trial. The short answer is that the Senate can hold a trial only when the House completes its share of the impeachment process outlined in the Constitution.

The Constitution’s impeachment provisions were modeled on 18th century British practice. Thomas Jefferson notes that in Great Britain, “The general course is to pass a resolution [in the House of Commons] containing a criminal charge against the supposed delinquent, and then to direct some member to impeach him by oral accusation, at the bar of the House of Lords, in the name of the Commons.” In other words, the House of Lords cannot hold a trial to convict or acquit someone until the House of Commons has informed the Lords that it has impeached that person.

This is the fundamental problem with a Senate-only trial. As a practical matter, the Senate does not know whether the House has impeached someone until it is told. Telling the Senate that it has impeached someone requires the House to take formal action explicitly for the purpose of communicating that fact and delivers the articles of impeachment specifying the high crimes and misdemeanors of which that person is guilty.

House Democrats are leveraging this fact of congressional life to pressure Senate Republicans to adopt more favorable trial rules. Republicans portray Pelosi’s strategy as inappropriate and constitutionally suspect. But both chambers attempt to influence each other’s internal proceedings on a routine basis. It is an inherent (and healthy) aspect of intra-branch bargaining.

While the Constitution gives the House and Senate plenary power over the procedures that govern their proceedings, the Senate is constitutionally barred from holding an impeachment trial until the House informs the Senate that it has impeached someone. The Senate’s own precedents concede that the House has adopted different procedures for impeaching someone, adopting articles of impeachment, and appointing managers to present those articles in the other chamber. By deciding to convict (or acquit) government officials of high crimes and misdemeanors on their own initiative, senators would, in effect, be usurping the House’s sole power to impeach.

Republicans should also consider what happens after they complete a Senate-only trial and the House sends over articles of impeachment. In that scenario, senators will be forced to either concede the irrelevancy of the prior trial, or ignore the articles of impeachment, thereby contradicting their own arguments that the Constitution, and the Senate’s rules, require the Senate to hold a trial when the House impeaches someone.

At best, a Senate-only trial is a mere messaging exercise with no legal effect. At worst, it weakens Congress’s bicameral structure and undermines the Constitution.

As proponents of unilateral action argue, senators should “honor the Constitution by holding a trial.” But they should hold a trial only when the Senate’s rules and the Constitution empower them to do so.