Who serves as the jury when a president is impeached?

Technically, impeachment is the Senate’s quasi-criminal proceeding instituted to remove a public officer, not the actual act of removal. Most references to impeachment, however, encompass the entire process, beginning with the House’s impeachment inquiry. The term will be used in that broader sense here. By design, impeachment is a complex series of steps and procedures undertaken by the legislature. The process roughly resembles a grand jury inquest, conducted by the House, followed by a full-blown trial, conducted by the Senate with the Chief Justice presiding. Impeachment is not directed exclusively at Presidents. The Constitutional language, “all civil officers,” includes such positions as Federal judgeships. The legislature, however, provides a slightly more streamlined process for lower offices by delegating much of it to committees. See Nixon v. US, 506 U.S. 224 (1993)(involving removal of a Federal judge). Presidential impeachments involve the full, public participation of both branches of Congress.

The Impeachment Process in a Nutshell

1.The House Judiciary Committee deliberates over whether to initiate an impeachment inquiry.

2. The Judiciary Committee adopts a resolution seeking authority from the entire House of Representatives to conduct an inquiry. Before voting, the House debates and considers the resolution. Approval requires a majority vote. 3.The Judiciary Committee conducts an impeachment inquiry, possibly through public hearings. At the conclusion of the inquiry, articles of impeachment are prepared. They must be approved by a majority of the Committee. 4.The House of Representatives considers and debates the articles of impeachment. A majority vote of the entire House is required to pass each article. Once an article is approved, the President is, technically speaking, “impeached” – that is subject to trial in the Senate.
3. The Senate holds trial on the articles of impeachment approved by the House. The Senate sits as a jury while the Chief Justice of the Supreme Court presides over the trial.
4. At the conclusion of the trial, the Senate votes on whether to remove the President from office. A two-thirds vote by the Members present in the Senate is required for removal.
5. If the President is removed, the Vice-President assumes the Presidency under the chain of succession established by Amendment XXV.

http://www.law.cornell.edu/background/impeach/impeach.htm