Key takeaways from the CRS report Impeachment and Removal (October 29, 2015, R44260) include:
The Constitution gives Congress the authority to impeach and remove the President, Vice President, and other federal “civil officers” upon a determination that such officers have engaged in treason, bribery, or other high crimes and misdemeanors.
A simple majority of the House is necessary to approve articles of impeachment.
If the Senate, by vote of a two-thirds majority, convicts the official on any article of impeachment, the result is removal from office and, at the Senate’s discretion, disqualification from holding future office.
The Constitution does not articulate who qualifies as a “civil officer.” Most impeachments have applied to federal judges. With regard to the executive branch, lesser functionaries—such as federal employees who belong to the civil service, do not exercise “significant authority,” and are not appointed by the President or an agency head—do not appear to be subject to impeachment. At the opposite end of the spectrum, it would appear that any official who qualifies as a principal officer, including a head of an agency such as a Secretary, Administrator, or Commissioner, is likely subject to impeachment.
Impeachable conduct does not appear to be limited to criminal behavior. Congress has identified three general types of conduct that constitute grounds for impeachment, although these categories should not be understood as exhaustive: (1) improperly exceeding or abusing the powers of the office; (2) behavior incompatible with the function and purpose of the office; and (3) misusing the office for an improper purpose or for personal gain.
The House has impeached 19 individuals: 15 federal judges, one Senator, one Cabinet member, and two Presidents. The Senate has conducted 16 full impeachment trials. Of these, eight individuals—all federal judges—were convicted by the Senate.