The 'decriminalisation' scheme, where wardens will work for the council not the police, is one of Labour's big ideas.
But Plaid spokesman Councillor Neil McEvoy said it could be a disaster if wardens were paid bonuses for the number of cars they ticketed.
He said: 'We are opposed to privatisation of parking and the repeat of the poll tax that will result.'
Plaid Cymru supports expanding residents' parking.
The Conservatives, on the other hand, oppose both. Spokesman Councillor Gareth Neale said: 'You have to be careful of expanding residents' parking as you can end up with overkill.'
Labour believes decriminalisation of wardens is an effective way of tackling parking problems.
Liberal Democrat spokesman Councillor Simon Wakefield said he also backed decriminalising parking.