Paris Mayor Anne Hidalgo is commissioning a study into making public transport free in the French capital to bring down grim levels of air pollution.
The study, announced late Monday, will see lawmakers and experts report back by the end of 2018 on whether the scheme would be financially feasible.
The Socialist mayor also said she has further negotiated discounts at park-and-ride stations to encourage commuters to leave their cars on the outskirts.
Paris could build a transport policy around free public transport "in which polluting cars no longer play a central role," she told Les Echos newspaper.
Hidalgo has made a priority of tackling smog and is planning stricter rules aimed at phasing out diesel cars by 2024, when Paris will host the Summer Olympics.
The study will also look at the possibility of introducing a toll, like London's congestion charge, as she seeks to discourage motorists from driving into Paris.
But her announcement prompted accusations of electioneering from opponents at a time when she is under fire over her management of the city, particularly on transport.
Her flagship policy of banning cars along a stretch of the River Seine was overturned by a court ruling last month which she has been forced to appeal.
And the city's popular Velib hire bikes have been virtually absent from the streets since January due to a botched handover to a new contractor.
Valerie Pecresse, the conservative head of the wider Paris region, said she was "open to all new ideas" but warned Hidalgo against "going it alone" on free transport.
Ticket sales bring in three billion euros ($3.7 billion) a year, Pecresse told Radio Classique, adding she would not settle for "a euro less".
Julien Bargeton, a senator from President Emmanuel Macron's Republic on the Move (LREM) party, said the announcement amounted to a "campaign launch" for 2020 municipal elections.
He accused Hidalgo of "policy zig-zags", while the rightwing Republicans blasted the free transport study as "irresponsible".
"While her policies are leaking all over the place -- dirty streets, public services under threat, rising taxes and municipal charges, court rulings -- Anne Hildago is trying diversion tactics," the party said.
The Communist party, however, backed free transport, saying it had already proposed the idea for young people and low-income Parisians.
Paris has previously offered free public transport during particularly bad spells of pollution in a bid to get people to leave their cars at home.
Hidalgo's proposals come as cities around the world weigh how to reduce pollution blamed for an estimated nine million deaths globally a year, according to a 2017 study in The Lancet.
Last month Germany made headlines with revelations it had considered nationwide free public transport, but Berlin cautioned there were no concrete plans to introduce such a policy.