SOON after landing a job at a Manhattan law firm nearly 20 years ago, Sara Horowitz was shocked to discover that it planned to treat her not as an employee, but as an independent contractor.

“I saw right away that something wasn’t kosher,” Ms. Horowitz recalls. Her status meant no health coverage, no pension plan, no paid vacation — nothing but a paycheck. She realized that she was part of a trend in which American employers relied increasingly on independent contractors, temporary workers, contract employees and freelancers to cut costs. Somewhat bewildered, somewhat angry, she and two other young lawyers who were also hired as independent contractors jokingly formed what they called the “Transient Workers Union,” with the facetious motto, “The union makes us not so weak.”

Ms. Horowitz’s grandfather was a vice president of the International Ladies’ Garment Workers’ Union, and her father was a labor lawyer. So it was perhaps not surprising that she responded to her rising outrage by deciding to organize a union. What she organized, however, was a newfangled version. The Freelancers Union, with its oxymoronic name, is a motley collection of workers in the fast-evolving freelance economy — whether lawyers, software developers, graphic artists, accountants, consultants, nannies, writers, editors, Web site designers or sellers on Etsy.

Today, the Freelancers Union is one of the nation’s fastest-growing labor organizations, with more than 200,000 members, over half of them in New York State. Ms. Horowitz, who has never lacked audacity, says she expects to expand the organization to one million members within three years. For some perspective, the United Automobile Workers union currently has 380,000 members. Of course, while hundreds of thousands of auto jobs have disappeared, the country is awash in freelancers and other independent workers. Studies by the Bureau of Labor Statistics and the Government Accountability Office show that there are more than 20 million of them. Many companies, including The New York Times, employ these workers.