Why President ratings with Congress (cq) drop
Because the President will often loose congress to opposition party in second term. e.g. Bush and Clinton.
National crisis like recession and Iraq war-Bush-will leave the public in a state of national panic and they will often turn to the president for blame so Congress may follow suit in order to coincide with popular opinion.
Clinton had ambitous and controversial ideas e.g. health reform. Congress did not like his plans and so voted against him.
What circumstances encourage Congress support