The Transport Workers Union (TWU) of America launched a statewide coalition against the scheme in Ohio on the grounds of safety.
TWU president, John Samuelsen, said: "We’re not going to let anybody give our jobs away to robots. We’re just not.
“Bus operators are too important to the safety of riders and the public, and these working-class jobs are too important to working-class families.
"If the politicians and profiteers try to wipe out these jobs, they will have a massive fightback on their hands.
"Our fight is to make sure that technology is used to improve safety and security, while maintaining a human presence on the passenger vehicles."
Members of the American Federation of Teachers and the Amalgamated Transit Union are backing the TWU campaign.
The ITF’s future of work coordinator, Mac Urata, added: "This shows how important it is that workers and their unions are involved in any plans to introduce automation of transport work.
"Workers are not opposed to all technology, but they are experts in their field. They know best what will and will not work. Any scheme is doomed to fail if it doesn't involve them from the start."
The TWU has produced this film about its campaign.