Froomkin has a 5 page article up at the WP. Go read it, you’ll be glad you did.

An excerpt:

It’s true that the Constitution grants the president unlimited clemency and pardon power. But presidents have generally used that power to show mercy or, in rare cases, make political amends — not to protect themselves from exposure.

The Framers, ever sensitive to the need for checks and balances, recognized the potential for abuse of the pardon power. According to a Judiciary Committee report drafted in the aftermath of the Watergate crisis: “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.”