I think Lincoln could have been impeached for a number of things (e.g. suspending habeas corpus in states not in rebellion, shutting down newspapers, property confiscation) but there wasn't much interest in impeachment or challenging the constitutionality of Lincoln's previously-unused 'new' powers. The country had enough on its hands without going after the president. In fact, though, several things Lincoln did were determined after the war to be unconstitutional (i.e. property confiscation and requiring people to pay taxes in person).