The President of these United States of America has the last say in any Federal bill that is passed in this country.So if we have a Pro-Gay/Anti-2nd amendment/Pro-Illegal Congress/Senate and they pass one of those type of laws(even by a slim majority), if the President is Pro-Life/Pro-2nd Amendment/Anti-Illegals, that bill has zero hope of passing.
Care to take a guess as to what would happen if the President agreed with Congress/The Senate? Might want to brush up on your Politics.