The Disclosure Act requires corporations, unions and big donors to reveal their identities in political ads. It was drafted after the U.S. Supreme Court ruled businesses and unions cannot be barred from certain types of political spending.
Rep. Chris Van Hollen, D-Md., the bill's sponsor, had hoped for a vote before the recess, The Hill reported. Some supporters fear giving lobbyists at least a week more could cost the bill some votes, although Van Hollen remained publicly optimistic.
Bridgett Frey, a Van Hollen spokeswoman, said the act has bipartisan support.
Those lobbying against the bill include the National Rifle Association, anti-abortion activists and the Chamber of Commerce.
The vote was delayed because of ongoing negotiations between the House and Senate over a tax, healthcare and jobs bill.