A U.S. Senate proposal to raise the level at which banks are deemed systemically important could help free up as much as $66 billion in capital at 11 lenders and allow for increased shareholder payouts.

If lawmakers approve the most extensive rewrite of the 2010 Dodd-Frank Act, it would remove an obstacle to returning capital for firms such as American Express Co., which has the largest percentage buffer over required minimums, and Capital One Financial Corp., which would have the most additional capital on a dollar basis, according to data compiled by Bloomberg.

The Senate Banking Committee is scheduled Thursday to debate a bill by Chairman Richard Shelby, an Alabama Republican, that would exclude banks with less than $500 billion of assets from being automatically designated as systemically important financial institutions, or SIFIs. If the measure becomes law and the Federal Reserve adopts similar limits in its annual stress test, regional and specialty-finance lenders could be freed from some of the toughest and most costly regulatory burdens.