While the courts exercised rulemaking powers granted to them under the Act without Congressional intervention for nearly forty years, Congress refused to allow the Federal Rules of Evidence to go into effect after their approval by the Supreme Court in 1973. The Rules of Evidence were eventually passed, with substantial changes, as legislation by Congress. Because of Congress's intervention in 1973 and subsequent years, the rulemaking powers granted to the judiciary by the Act have been reduced, causing the Act to command less importance in recent years. However, the Act still prevents litigants from challenging the validity of constitutional Federal Rules via the Erie Doctrine.
by Peter G. McCabe (American University Law Review, v.44 #5, June 1995).