Uber and Postmates filed a lawsuit in federal court in California on Monday, seeking an injunction to prevent the state’s landmark freelancer law from taking effect against them on Jan. 1 as scheduled. The action underlines how high the stakes are for Uber and Postmates with the new California law, called Assembly Bill 5. The law could potentially threaten their businesses because under it, workers must be classified as employees rather than contractors under certain conditions, such as if a company controls how they do their work or if the work is a regular part of the company’s business.