Article I, Section 3, Clause 6:
The Senate shall have the sole Power to try all Impeachments. When sitting for that Purpose, they shall be on Oath or Affirmation. When the President of the United States is tried, the Chief Justice shall preside: And no Person shall be convicted without the Concurrence of two thirds of the Members present.
The federal impeachment process stems originally from English practice, 1 where the House of Commons could impeach individuals and the House of Lords would convict or acquit. 2 Most of the American colonies and early state constitutions adopted own impeachment procedures before the establishment of the federal constitution, with the power to try impeachments located in various bodies. 3 At the Constitutional Convention, the proper body to try impeachment posed a difficult question. 4 A number of proposals were considered that would have assigned responsibility for trying impeachments to different bodies, including the Supreme Court, a panel of state court judges, or a combination of these bodies. 5 One objection to granting the Supreme Court authority to try impeachments was that Justices were to be appointed by the President, calling into question their ability to be independent in an impeachment trial of the President or another executive official. 6 Further, a crucial legislative check in the Constitution's structure against the judicial branch is impeachment, as Article III judges cannot be removed by other means. 7 To permit the judiciary to have the ultimate say in one of the most significant checks on its power would subvert the purpose of that important constitutional limitation. 8 Rather than allowing a coordinate branch to play a role in the impeachment process, the Framers decided that Congress alone would determine who is subject to impeachment. This framework guards against, in the words of Alexander Hamilton,
a series of deliberate usurpations on the authority of the legislature by the judiciary as Congress enjoys the power to remove federal judges. 9 Likewise, the Framers' choice to place both the accusatory and adjudicatory aspects of impeachment in the legislature renders impeachment
a bridle in the hands of the legislative body upon the executive branch. 10
The Framers' choice also imposed institutional constraints on the process. 11 Dividing the power to impeach from the authority to try and convict guards against
the danger of persecution from the prevalency of a fractious spirit in either body. 12 Likewise, the requirement of a two-thirds majority in the Senate to convict and remove an official ensures (at least in the absence of one political faction gaining a supermajority) that impeachment and removal is not a strictly partisan affair and is limited to situations where consensus is possible. 13
Finally, the Framers made one exception to the legislature's exclusive role in the impeachment process that promotes integrity in the proceedings. While the Presiding Officer of the Senate (typically the Vice President of the United States) usually presides at impeachment trials, the Chief Justice of the Supreme Court presides in the event that the President of the United States is tried. 14 This provision ensures that a Vice President shall not preside over proceedings that could result in his own elevation to the presidency, a particularly important concern at the time of the founding, when Presidents and Vice Presidents were not elected on the same ticket and could belong to rival parties. 15