Republican Sen. Tom Coburn of Oklahoma, who sponsored the proposal with Democratic Sen. Mark Udall of Colorado, says taxpayers should not have to contribute, as they are doing for the two conventions this year, when the nominee has been decided.
Coburn says the public this year is spending more than $36 million for the conventions.
The measure, if it becomes law, would not take effect until future political conventions. The idea would need the House's OK.
The Senate vote was 95-4 for the proposal, which was presented as an amendment to a farm bill.