A judge with the Maryland Public Service Commission proposed Thursday to categorize Uber and other popular rideshare companies as "common carriers" — and subject them to the same regulations as taxis.

Uber has threatened to leave the state if the commission votes next month to classify as a cab company. That would be a victory for traditional cab companies, who have argued that Uber, a smartphone app that connects passengers with drivers, enjoys an unfair advantage.

Judge Terry J. Romine wrote that Uber is a "public service company" because it offers rides for hire — and so should be subject to the jurisdiction of the commission.

Romine, the state's chief public utility law judge, said the company should file an application to operate as a for-hire carrier in the city.

If the commission approves Romine's proposal, a spokeswoman for Uber said, the company will appeal.

The spokeswoman, Natalia Montalvo, said it would be disappointing if the commission relied on "antiquated regulations to restrict transportation options for riders" and "limit economic opportunities for drivers."

"We are confident that on appeal reasonable standards that promote competition and choice will prevail," she said in a statement.

Dwight R. Kines, a vice president for Veolia Transportation, said he was pleased that Romine recognized Uber "for what they are."

"They do the exact same thing any other common carrier does," said Kines, whose business owns the Yellow Cab taxi company. "Uber, Lyft, Uber X ... all those companies are just a different way of booking a trip."

Uber and Lyft, which also operates in Maryland, both allow customers to order a ride directly from a driver and pay for it with a smartphone app.

They and other companies have faced legislative battles in several cities as taxi companies and regulators have raised concerns regarding safety, competition and the qualifications of the drivers and cars.

Uber supported state legislation this year that would have required background checks, vehicle inspections and rideshare insurance of up to $1 million. The bills, which would have allowed Uber to continue calling itself a smartphone app, and not a cab company, failed.

One sponsor of the legislation said any decision that would push the company out of the state would be "a great detriment to consumers."

"We'd be first state in the nation to have Uber pull out," said Del. Ben Barnes, a Prince George's County Democrat. "I think that would be a big mistake."

In Seattle, the companies have agreed to ensure that drivers are insured any time they're using the apps.

Officials there grew concerned after a 6-year-old was struck and killed in San Francisco last New Year's Eve by a driver with Uber X. Uber X, the company's lowest-cost service, matches riders with drivers who use their personal vehicles.

Nebraska's Public Service Commission sent letters to Uber and Lyft warning them that they would need permits to operate in the state. The Chicago City Council is considering an ordinance that would require drug tests, background checks and training for drivers, inspections for vehicles and license fees.

Uber and Lyft, which both launched in San Francisco and have spread quickly across the country, advertise low prices and fast service. Lyft nearly doubled its footprint on Thursday by expanding into 24 more cities, bringing its total to 60. Uber says it offers rides to people in more than 70 cities.

Aaron Heinsman calls himself an Uber convert. The 42-year-old Mount Vernon man has lived in Baltimore for more than a decade without a car. He once relied on taxis, and later, ZipCars. Now he uses the smartphone app.

If Uber were to leave the state, Heinsman said Thursday, he might miss more early-morning flights — the service has saved him in a pinch when taxis have arrived late or not at all.