Some time back Eugene Volokh had a post on VC in which he discussed whether the CBS forgeries were illegal. He found some obscure laws that might apply, but pretty much concluded that they were not illegal, and would not have been even if CBS had typed them up themselves.
The extent to which the Kerry campaign is involved in this is still being discovered, but let's turn the situation into a hypothetical for discussion.
Bushgate: President Bush, believing that rival John Kerry aided the Communists in 1970, tells staffers to forge documents as proof, and to leak them to CBS News. CBS News reports them as genuine, but immediately bloggers prove that the documents are forgeries, and within a week Bush's involvement also becomes known....
...Compare this with two earlier episodes.
Watergate. President Nixon's re-election decide to break into Democratic Party offices in the Watergate building to bug phones, but are immediately caught. President Nixon approves of plans to cover up the involvement of top campaign officials in the burglary.
Monicagate. President Clinton, under investigation for financial irregularities, lies under oath when asked about his sexual liason with staffer Monica Lewinsky.
In Bushgate, no law has been violated, unlike in Watergate (burglary, obstruction of justice) or Monicagate (perjury). Yet which is the most serious? Bushgate.
The Watergate burglary was a minor crime, and ordinarily the burglars would have received a light punishment-- perhaps none at all. The judge was severe because he thought there were higher-ups involved, and higher-ups did try to conceal who was involved. Impeachment charges were brought against Nixon not because the burglary or obstruction of justice was harmful in itself (or would have been even if successful), but because a President who obstructs justice in a minor case like this lacks moral legitimacy and would probably commit more serious crimes if given the opportunity.
Similarly, President Clinton's perjury was a minor crime, but a crime nonetheless, committed for personal advantage, and revealing a disregard for the law improper in a President.
In Bushgate, I think it would be appropriate to impeach President Bush for the forgeries. He would not have shown disregard for the law-- that kind of forgery is not a crime-- but he would have shown low enough moral character that we would not want him as President. Moreover, unlike in Watergate and Monicagate, the forgery would be important in itself, a clear threat to honest elections.
This is one good argument against the peculiar notion floated during the Clinton Impeachment that a President should only be impeached if he has committed a crime, and a serious crime at that. No-- impeachment is a procedure for quickly getting rid of Presidents who have shown themselves unfit for office,especially if their unfitness consists in trying to use unfair means to expand their own power. This was also why Andrew Johnson should have been impeached: it was not that he had violated an act of Congress concerning the firing of Cabinet officers, but that he had tried to thwart the legislative branch's laws about Southern Reconstruction wherever he could.