"When, then, a law is in its nature a contract, when absolute rights have vested under that contract, a repeal of the law cannot divest those rights; and the act of annulling them if legitimate, is rendered so by a power applicable to the case of every individual in the community."

Marshall then defines a contract as a compact between two or more parties, and is either executory or executed. http://www.history1700s.com/page1760.shtml

The government is, then similar to a corporation. The government has vested interest in protecting the rights of the people it serves. Corporations are in the business to be of public service. Corporations rely upon its charter to protect not only itself but also the people it serves. The same situation exists for the congress.

Marshall continues to explain further that a legislative law is a contract and thus as a contract it cannot be broken.

Link http://www.justice.gov/usao/eousa/fo...4/civ00059.htm