he shall have Power to Grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment. What does that really mean? The actual impeachment process by the house and senate are not "offenses" against the US. So it's obvious no pardon can be issued to stop them, or nullify their effect. And why would the constitution have to state the crimes for which they were impeached, can still be prosecuted, ie not subject to "double jeopardy" Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.