I don't think there is any question that he has repeatedly violated the US Constitution...virtually the entire federal government operates outside the limits imposed on it by the Constitution...it certainly did not start with President Bush (I think every President since Cleveland has ignored the Constitution) and contempt for the Constitution is certainly demonstrated in practice by virtually every member of Congress (Ron Paul may be the only exception)...but none of those facts gives license to violate your oath of office.
And I do believe repeated violations of the Constitution is an impeachable offense...but, of course, who would impeach him...his partners in crime in Congress?
I think conservatives who want to impeach this President are on the fringe. So far to the right that they end up standing shoulder to shoulder with Russ Feingold.