The nation's largest chain of rehabilitation hospitals yesterday was accused of sweeping accounting fraud stretching back almost to the company's inception, in a case that regulators said might rival wrongdoing at Enron and WorldCom. The Securities and Exchange Commission said that HealthSouth and Richard M. Scrushy, 50, the company's flamboyant founder and chief executive, falsified profits and assets for decades in a scheme to delude Wall Street investors and protect Mr. Scrushy's own large holdings of HealthSouth stock. The chief financial officer of HealthSouth has already agreed to plead guilty to fraud and to cooperate with investigators, who estimate the company has added at least $1.4 billion in nonexistent earnings to its profits since 1999 and inflated company assets by $800 million to cover up fraud. ''HealthSouth's fraud represents an appalling betrayal of investors,'' Stephen M. Cutler, the S.E.C.'s director of enforcement, said.