Uber’s battle to defend its business model amped up this week with a report that New Jersey has declared the company’s drivers are misclassified as independent contractors, and is demanding $650 million in unpaid unemployment and disability taxes.

Already fighting accusations on the legislative and litigation fronts that its drivers are misclassified as independent contractors, the New Jersey action is a harbinger of an increasing focus on Uber’s practices by state and federal taxing authorities, said Shannon Liss-Riordan, the Boston lawyer who brought misclassification suits against the ride-hailing company in Massachusetts and California.