The problem with that assertion is that it is neither factual nor an acceptable exercise in logic. Why? Because it arbitrarily assumes that the only means by which Congress can reject an action of the President that it doesn't like is by impeaching him - a logical absurdity on its face, especially given that only two presidents have ever been impeached. Applying your argument consistently one could presume upon this latter fact that only two presidents have ever been substantively opposed by Congress on anything, and we know that proposition to be inherently false.
I love it when you guys try to use 'logic'!
If what Lincoln was doing was unconsitutional they should have impeached him-period.
The Separation of powers was made that each branch would be jealous of its power.
Clearly Congress did not feel threatened by Lincoln's actions.
The fact that only two Presidents have been impeached is irrevelent, that was the power that Congress had and did not use.