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.

U.S. Constitution, Article I, Section 4, Clause 7.
Since impeachment does not impose criminal penalties nor guilt under the law, it is not subject to presidential pardon. Only if Bush were, after
removal from office, indicted in a criminal court and convicted of a crime, would a pardon become possible.