Bush gives up

Bush commuted Libby's sentence out of fear that he would cooperate with the prosecution to give up the involvement of Bush and Cheney in the outing of Plame and retaliation against Wilson.
A report written and released by the Judiciary Committee in 1974 in the aftermath of the Watergate crisis contained the following statements:

In the [Constitutional] convention George Mason argued that the President might use his pardoning power to "pardon crimes which were advised by himself" or, before indictment or conviction, "to stop inquiry and prevent detection." James Madison responded:

[I]f the President be connected, in any suspicious manner, with any person, and there be grounds [to] believe he will shelter him, the House of Representatives can impeach him; they can remove him if found guilty...

Madison went on to [say] contrary to his position in the Philadelphia convention, that the President could be suspended when suspected, and his powers would devolve on the Vice President, who could likewise be suspended until impeached and convicted, if he were also suspected.

full context here