text: Simple violation of the Constitution isn't enough for impeachment.  Presidents "violate" their duties under the Constitution all the time.  Bush's refusal to enforce through the EPA the Clean Air Act comes readily to mind. The Constitution is rather specific in what can be used for impeachment as, " for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors."
  While the term "high crimes and misdemeanors" are a bit of a gray area SCOTUS long ago determined that the President (and Congress) must be provided some wiggle room to operate and this sometimes means violating the law, including the Constitution, if only because of competing duty to govern with and within the duty of "preserve, protect and defend the Constitution."
  This doesn't mean that the President is above the law.  Perhaps to put it too simply, Congress and SCOTUS will look favorably on excusable errors that violate the law.