One of the key problems in impeaching Bush is the question of what to impeach him for - his list of misdeeds, lies and political crimes is so long it will be difficult to pick a single issue out of the mix to focus on. Elizabeth Holtzman (the author of the FP piece) picks two. The strongest legal case is the warrantless NSA wiretapping. The strongest political case is the war in Iraq, specifically the claim about uranium from Niger. It all comes down to the clause from article 2, section 3 of the Constitution:
he shall take care that the laws be faithfully executedDifficult to do when you have John Yoo legitimizing that voice inside your head that tells you you're above the law.