President Biden’s nominee to head the Department of Labor is facing headwinds in the Senate before her confirmation hearings even begin, not just from Republicans and their allies in the business world, but also from moderate Democrats.

This week, a coalition of small-business owners, contract workers, freelancers, and restaurant workers who rely on tips kicked off a campaign against Julie Su’s nomination to head the labor department with a series of billboards and ad buys in states such as Maine, Arizona, West Virginia, and Montana that are served by potentially persuadable Democratic senators.

The “Stand Against Su” campaign aims to convince the senators that Ms. Su is hostile to small business and a threat to the livelihoods of independent contractors across the country. “Throughout her career Su has been a fiery critic of capitalism, which she believes is a system rooted in exploitation and racism. Her hatred of capitalism is evident in her anti-business rhetoric and record,” the group said.

When he nominated her in late February, Mr. Biden held her up as a champion of workers and a critical partner to the man who would be her predecessor in the job, Marty Walsh. “Julie has spent her life fighting to make sure that everyone has a fair shot, that no community is overlooked, and that no worker is left behind,” Mr. Biden said. “Over several decades, Julie has led the largest state labor department in the nation, cracked down on wage theft, fought to protect trafficked workers, increased the minimum wage, created good-paying, high-quality jobs, and established and enforced workplace safety standards.”

Her critics say she has been nothing of the sort for one group: gig workers and freelancers who want to earn a living without being tied to companies as employees. When she was head of California’s main labor agency in 2019, she was a prime force behind California’s Assembly Bill 5, which tried to force many companies such as Uber, Lyft, and others in the so-called gig economy that rely on independent contractors to reclassify those workers as full-time employees with full benefits.

After outcries from sectors including freelance journalists and jazz musicians to accountants and real estate agents, more than 100 professions were exempted from the law. A number of business groups representing, among others, truck drivers and press photographers have sued to block the law after their workers were told by clients that because of the law they would no longer be able to work in the state of California. Ms. Su championed the law as a “model for the country.”

Critics have also rounded on Ms. Su’s management of California’s unemployment insurance during the Covid pandemic, during which some $40 billion in fraudulent claims were paid out with little oversight. A state audit blamed her lax leadership for the boondoggle.

When she was confirmed as Mr. Walsh’s deputy two years ago, she barely eked out a win with a party line 50-47 vote. She enjoys the vocal support of almost all major labor unions and liberal Democrats in the Senate, among them Senator Wyden of Oregon, who called her an “excellent choice” for the position. 

“Throughout her career she has advocated for workers, especially the most vulnerable workers,” Mr Wyden said when she was nominated. “Whether it was fighting wage theft and forced labor or ensuring safe working conditions, Deputy Secretary Su has always stood firmly on the side of the worker.”

With a razor-thin majority in the Senate that depends on the vote of two independents — Senators Sinema of Arizona and King of Maine — and one notoriously unreliable vote from Senator Manchin of West Virginia, Ms. Su’s nomination is far from certain. Senator Hickenlooper of Colorado told the Hill this week that getting her over the finish line will be an uphill challenge. “I think it’s going to take some work,” he said.

If Ms. Su is not confirmed, it would mark the third high-profile nomination by the Biden administration to be torpedoed by moderate senators in recent weeks. Mr. Biden’s nominees to head the Federal Aviation Administration and the Federal Communications Commission were both forced to withdraw after failing to garner enough Democratic support.