Article I, Section 8, § 18:(The Congress shall have power:) "To make all laws which shall be necessary and proper for carrying into Execution the forgoing powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Office thereof."
The necessary and proper clause comes at the end of Congress' ENUMERATED powers-those listed in Section 8. It gives Congress the power to make laws with which to carry out those powers. For example: Congress has the power to coin money, but nothing in that "power" tells Congress HOW to do that. The necessary and proper clause constitutes Congress' GENERAL powers.
The necessary and proper clause complicates American politics by giving Congress the power to do, basically, ANYTHING it considers to be NECESSARY AND PROPER. That could be anything from increasing Social Security, to building a bridge to nowhere.
The CHECK on this power (and the cause of some of the conflict) is the VETO power of the President. If he/she doesn't agree with the proposed legislation, the bill is vetoed and returned to congress. Congress may override the veto, but that requires 2/3 of both Houses.
All of this, of course, can result in political arguments on both sides of the aisle, as well as infighting between the members of either party.
Hope this helps.