Vested rights doctrine
Vested rights doctrine protects someone who won a legal decision from a legislature seeking to overturn the decision. The doctrine has two components.
First, it protects the property right obtained by the victorious party against a taking of that property by the legislature. In this sense the protection is similar to constitutional protection of contractual rights by the Contract Clause .
Second, and equally important, "vested rights" doctrine safeguards separation of powers against overreaching by the legislature. "[C]onsistent with the separation of powers, it protects judicial action from superior legislative review, 'a regime [that would be] obviously inconsistent with due process of law and subversive of the judicial branch of government.'" Georgia Ass'n of Retarded Citizens v. McDaniel, 855 F.2d 805, 810 (11th Cir. 1988) (quoting Daylo v. Administrator of Veterans' Affairs, 501 F.2d 811, 816 (D.C. Cir. 1974)).
"Vested rights" doctrine was first announced in McCullough v. Virginia, 172 U.S. 102, 123-24 (1898): "It is not within the power of the legislature to take away rights which have been once vested by judgment. Legislation may act on subsequent proceedings, may abate actions pending, but when those actions have passed into judgment the power of the legislature to disturb the rights created thereby ceases."