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.

Bloomberg called the measure a well-intended but misguided effort that would create more lawsuits than jobs.

“Hiring decisions frequently involve the exercise of independent, subjective judgment about a prospective employee’s likely future performance,” he said in a statement.

And unlike other characteristics that employers are generally banned from considering, such as an applicant’s race, religion or gender, “the circumstances surrounding a person’s unemployment status may, in certain situations, be relevant to employers when selecting qualified employees,” he said.

Business groups say that no-unemployed-applicants-need-apply ads represent a tiny fraction of the millions of job openings nationwide each year.

One 2011 listing that got city lawmakers’ attention — it required that applicants for an opening as a New York legal secretary “must be currently employed” — was mistakenly written that way, said William Alcott, a lawyer for the firm that posted it, McGuireWoods LLP.

“It was not our policy then and isn’t our policy now,” he said this week.

Like other measures that have passed, the New York City one would ban help-wanted ads that say unemployed applicants won’t qualify. It would also more generally prohibit employers from refusing to hire candidates because they are out of work.

But New York’s measure would go further than the others by letting rejected applicants sue employers for damages.

Companies see it as government meddling and “creating another basis for unmerited lawsuits against employers,” said Kathryn Wylde, president of the Partnership for New York City, an influential business group.

President Barack Obama proposed in 2011 to make it illegal to refuse to consider unemployed applicants.

New Jersey in 2011 became the first state to outlaw the practice. The state Labor Department has gotten one complaint so far and cited a company for an ad that excluded jobless applicants; the case is not yet resolved, the agency said this week.

Oregon and the District of Columbia followed suit last year, while 15 other states considered similar proposals, according to the National Conference of State Legislatures. Gov. Jerry Brown vetoed such a measure in California last fall, indicating he wasn’t happy with changes made to it.