Because his argument has almost no historical basis, he makes allowances that ultimately render it absurd.
Alan Dershowitz has been widely taken to be saying that a president can do anything he feels necessary to aid his reelection and not be impeached for it; since the president will always feel that his reelection is in the public interest, it cannot be a corrupt motive. He has, however, protested at this characterization. He has a tiny point. As far I can tell, here is what he is actually saying: The president can be impeached for treason, bribery, crimes (criminal violations of statutes), and “crime-like” actions. “Abuse of power” is not impeachable unless it falls into one of those categories.
So it’s not true, on Dershowitz’s view, that the president can do anything to aid his reelection without being subject to impeachment. He can’t commit a crime (or “crime-like” act) toward that end. The key point in his argument is that “abuse of power” is not a freestanding impeachable offense. The point he made about presidential motives is surplus to his argument.
The trouble is that the key point is incorrect. There’s almost nothing in the Founding-era debates or the historical practice of impeachment that suggests that only violations of statutes count as high crimes and misdemeanors and thus as impeachable. Hence the need for the qualifier that “crime-like” activities count too. But this qualifier destroys the Dershowitz position. The supposed danger of allowing impeachment for abuse of power — a danger that can’t be acceptably diminished by the two-thirds-of-senators requirement for conviction, the ability of voters to reject those senators at the next election — is that it’s vague and malleable. It’s not as though “crime-like” is capable of precise definition.
Dershowitz is right, of course, to posit that an action can have multiple motives. The president’s supporters correctly note that if any trace motive of political self-interest makes an action an abuse of power, then nearly anything any president has ever done would be impeachable. His opponents correctly note that if the president’s mere belief that he’s a good president counts as a proper motive for official action, then impeachment for abuse of power is nearly categorically impossible.
The answer can’t be that anything over 50 percent self-interested is impeachable and anything less than that is fine. It has to be that it’s up to members of Congress to use their best judgment without some made-up legal standard that gives that judgment spurious precision. It’s up to voters to make their best judgment, too, in judging the senators’ decision.
Madison’s example of the abuse of the pardon power is again clarifying. What if the president encouraged his allies to commit a crime to benefit himself and then pardoned them for that crime? That pardon would not necessarily have violated any statute, especially given the small federal government at the time of the Founding. The pardon power itself is nearly unreviewable, and maybe wholly unreviewable. Yet Madison believed that such a case would demand the president’s impeachment and removal. At no point did he raise the possibility that this course of conduct should not be impeachable because the president might have embarked on it to secure his power and consider that goal a public necessity — probably because it would have been a ridiculous thought.