The US Chamber of Commerce sued Seattle yesterday, objecting to that city's recent passage of a law allowing Uber and Lyft drivers to unionize.

It's the latest round in a growing battle over whether workers in the on-demand app economy should be treated as employees or independent contractors. The Chamber of Commerce lawsuit (PDF) says they're contractors and therefore can't unionize under the National Labor Relations Act. The Chamber claims that the Seattle law also violates the Sherman Antitrust Act.

"If allowed to stand, Seattle's Ordinance would threaten one of the most vibrant, cutting edge sectors of the economy," Chamber lawyers write. With 40,000 local governments in the US, thousands of "separate and independent collective bargaining regimes" could "undermine the flexibility, efficiency, and choice that accompany independent driver arrangements."

Drivers with ride-sharing apps "choose when they work, choose where they work, and operate in their own vehicle," Chamber lawyers write. "Drivers are not required to work any particular amount of time each week... [and] may, if they choose, contract with more than one service."

"It’s no coincidence that, in the 126 years since the passage of the Sherman Antitrust Act and the 81 years since the passage of the National Labor Relations Act, the unions are only now attempting to impose a local regulatory scheme to organize independent contractors," said the Chamber's chief legal officer, Lily Fu Claffee. "This has never been tried before, because it is clearly inconsistent with federal antitrust and labor laws."

For its part, Uber has said the lawsuit "raises serious questions" about Seattle's actions, according to a statement provided to Geekwire.

The employee versus contractor debate may come to a head this summer, as a lawsuit over Uber drivers' status is scheduled for trial in San Francisco. But that's not the only high-stakes "sharing economy" labor dispute currently pending. The International Brotherhood of Electrical Workers has also filed a petition at the National Labor Relations Board claiming Uber drivers are employees under NLRA, and that union is seeking to represent them.

The ride-sharing unionization bill was passed 8-0 by the Seattle City Council in December, but the city's mayor declined to sign it into law, saying it had "several flaws" including "unknown costs of administering the collective bargaining process" and the burden on city staff. The ordinance has yet to take effect, but Geekwire reports it will still become law even without the mayor's signature.