Bristol's Green Party wants a congestion charge for non-residents driving into the city centre.
The party says its proposed amendment for Bristol City Council's budget would begin in 2021 and raise £6.5m a year.
Mayoral candidate Sandy Hore-Ruthven said: "We're wanting to build more and more homes so congestion and air pollution is only going to get worse."
The ruling Labour Party says charging non-Bristol residents is illegal as councils cannot consult them.
A Labour spokesman added: "It cannot be enforced unless it equally applies to Bristol residents, when it would be a stealth tax.
"So, Green Party policy is illegal and yet another meaningless chase for cheap headlines. It's time for the Green Party to be honest."
The Green Party said its proposal was legally possible.
"It's not a stealth tax. If you live in Bristol, you pay your council tax, if you run a business you pay your business rate and you contribute to your transport infrastructure," said Mr Hore-Ruthven.
"For those who live outside the city they don't contribute.
"This proposal would bring in £6m a year for the buses which would be cheaper and more reliable and give people an alternative."
Prof Graham Parkhurst, director of the Centre for Transport and Society at the University of the West of England, said: "Maybe the council ought to look at the workplace council levy which was successfully introduced by Nottingham a few years ago.
"It specifically targets people who have got free city centre parking so most of them would never think of using public transport because they have free car park spaces and a free company car."
A vote on the proposal will take place at the council's budget meeting next week.