Article II, Section 2, Clause 1 of the U.S. Constitution

[H2O Man]

7. Ford's action

prevented ex-president Nixon from being tried in criminal court. Nixon was not really worried about facing criminal charges, as he was convinced that he could claim he needed classified documents, etc, for his defense, that could not be made public. It can be argued that he was wrong in this belief. But if he had been impeached, the Senate would have had access to the documents that a jury very likely would not have been allowed to see (thus, a mistrial).

Impeachment is restricted to the president, vice president, and federal judges. Those in Congress cannot be impeached, but rather, face criminal charges. Thus, a president can pardon those facing impeachment, but only for the criminal charges that could follow their removal from office. If someone is pardoned for the criminal charges before impeachment, they can still be impeached -- even if they have resigned from office. In Nixon's case, the House determined that it was not necessary to follow through, which negated the possibility of the Senate trying the case.