According to the Constitution: "...Treason, Bribery, or other high Crimes and Misdemeanors". Treason and Bribery are clear. The definition of "high Crimes and Misdemeanors" was left intentionally vague by the founders but is generally considered to refer to misconduct or a violation of the public trust that is injurious to society. They are not limited to statutory violations (breaking regular laws).
The general message from interpreters of the Constitution is that impeachable offenses are not limited to specific violation of criminal statutes. The contitution was intentionally vague on this point to allow flexibility in prosecuting a president. Justice Joseph Story wrote in his Commentaries on the Constitution in 1833:
"Not but that crimes of a strictly legal character fall within the scope of the power; but that it has a more enlarged operation, and reaches, what are aptly termed political offenses, growing out of personal misconduct or gross neglect, or usurpation, or habitual disregard of the public interests, various in their character, and so indefinable in their actual involutions, that it is almost impossible to provide systematically for them by positive law." http://www.impeachbush.tv/impeach/offenses.html
Seems to me there are more than enough grounds here.