This will be a brief diary, but I just want to correct a misunderstanding that's been going around a fair amount in the last two weeks.
First off:
A president can be impeached for essentially anything. All it takes—remarkably, IMO—is a simple majority vote in the House.
Two presidents have been impeached: Andrew Johnson and Bill Clinton. Richard Nixon would have been the third, in all likelihood, had he not resigned in disgrace first.
After the president is impeached by the House, a trial is held in the Senate. It then takes a 2/3 vote of the Senate to convict the president. A conviction is required to remove the president from office.
Neither Johnson nor Clinton was convicted, although Johnson was just one vote away.
Under the current political environment, it is very likely—possibly even probable—that Obama will be impeached by the House. There is essentially zero likelihood, barring the exposure of some sort of damning criminal action on Obama's part, that the Senate will actually convict and remove him from office.