NEW YORK — Help wanted. Qualifications: Must already have a job.

It's a frustrating catch for those out of work in an era of high unemployment: looking for a job, only to find that some employers don't want anyone who doesn't already have one.

But after four years of above-average joblessness in the U.S., efforts to bar such practices by employers have met with mixed results.

While New Jersey, Oregon and Washington, D.C., have passed laws making it illegal to discriminate against the unemployed, New York City's billionaire-businessman mayor vetoed on Friday what would have been the most aggressive such measure in the country. Similar proposals have stalled in more than a dozen other states and Congress.

Advocates for the unemployed say such hiring practices are unfair, particularly to those who have been laid off because of the economic crunch and not through any fault of their own. Businesses, though, say that the extent of such practices is exaggerated, hiring decisions are too complicated to legislate, and employers could end up defending themselves against dubious complaints.

Nationally, more than 1 in 3 unemployed workers has been looking for at least six months, according to the federal Bureau of Labor Statistics.

Janet Falk said that when she applied for a public-relations job at a New York law firm two years ago, the recruiter told her she wouldn't be considered because she had been out of work for more than three months. The recruiter was being paid to find candidates who were in jobs or just out of them.

"My personal view is that hiring is like musical chairs, and if only the people who are already on the dance floor are playing, then the long-term unemployed can't get in the game," said Falk, who was laid off four years ago. She now runs her own consulting business.

An October 2011 search of New York City-based job listings found more than a dozen that explicitly required candidates to be employed, Manhattan Borough President Scott Stringer's office said. A broader review that year by the National Employment Law Project found 150 ads that were restricted to or aimed at people currently working.

As for why, experts say employers may think that unemployed applicants' skills have atrophied, that they lost their jobs because of their own shortcomings, or that they will jump at any job offer and then leave as soon as something better comes along.

But "'don't apply, don't even try' is the opposite of American values," New York City Council Speaker Christine Quinn said when the measure passed last month. She said Friday that she expects the City Council will override Mayor Michael Bloomberg's veto within a month.