The arrest of almost 400 immigrant workers in a raid earlier this month of Agriprocessors, Inc., a meatpacking plant in Postville, Iowa, shows that something is wrong with our immigration system.

To be sure, the scale of the action, with warrants for almost 700 people, arrests of almost 400, and 300 employees actually charged with immigration violations and criminal charges such as identity theft and false use of Social Security numbers, shows a blatant disregard of the law.

But let’s dig deeper. Raids in Postville and at Swift & Company’s six meat processing plants in Marshalltown, Iowa, last December as well as raids in Colorado, Nebraska, Utah, Minnesota, and Texas, prove that Congress authorizes too few visas for foreign workers, far fewer than the number employers want and need to hire to keep their businesses running.

With a 5% national unemployment rate, the basic economic truth is that, in some sectors, employer demand for foreign workers exceeds the numbers that Congress, pressured by unions, will admit.

Indeed, the government issues so few visas, both for skilled and for unskilled labor, that entry permits are exhausted within days of becoming available.

By law, the government may issue 66,000 H-2B visas for temporary, unskilled, nonfarm workers, such as those employed by Agriprocessors. In 2005, Congress allowed H-2B visa requests for prior holders of the visas during any of the past three years not to count against the 66,000 limit. In 2006, 97,279 H-2B visa holders were admitted, a seven-fold increase from 14,345 visas a decade earlier.

Last year, however, Congress failed to renew this legislation, so in 2008 all H-2B visas will count against the 66,000 limit. As a result, this election year there are few visas available for people who had not previously been admitted.

Last week the U.S. Labor Department took a step toward modernizing the unskilled-visa process by proposing new rules.

Rather than having to apply twice for the H-2B visa, once to a state workforce agency and then to the Labor Department, an employer would apply only to the Labor Department. This would make the process faster and fairer to employers, because state agencies have different waiting times, penalizing certain employers. Comments on the proposed rules are due by July 7.

This streamlined process would be of even greater use if Congress were to increase the numbers of unskilled visas available.

Similar limits constrain visas for skilled labor, or H-1B visas. On April 1, the United States Center for Immigration Services began to accept H-1B visa applications for 2009. The congressional allowance of 65,000, plus 20,000 more H-1B visas for those with U.S.-awarded masters degrees, was filled within a week. The center plans to award visas by lottery.

A year ago, the quota for H-1B visas for 2008 was met on the very first day, when 150,000 H-1B visa applications poured in.

Yet H-1B visas for skilled workers have not always been in short supply. During the 1990s, Congress temporarily raised the quota to 195,000, a number that did not exceed demand, but in 2004 the quota reverted to 65,000, a miniscule portion of the U.S. labor force of 154 million.

Even if the quota were raised to 150,000 annually, that would be less than one tenth of 1% of the labor force. A 150,000 quota would still block admission to the vast majority of applicants.

When Congress in 2007 engaged in a lengthy and ultimately futile debate on immigration that produced no bill, the lawmakers left the system unchanged. However, both leading presidential candidates say it is broken and propose to fix it.

Senator Obama, the front-running Democrat, “supports a system that allows undocumented immigrants who are in good standing to pay a fine, learn English, and go to the back of the line for the opportunity to become citizens.” This was the essence of legislation sponsored by Senator McCain which failed last year.

Now, in the throes of the 2008 campaign, Senator McCain seems to have backed away from making more immigrants legitimate workers. He writes on his Web site, “If we have learned anything from the recent immigration debate, it is that Americans have little trust that their government will honor a pledge to do the things necessary to make the border secure.”

He does not mention allowing undocumented workers to pay a fine and get on a path to citizenship. One hopes that if he is elected he will return to his larger vision of how to fix the system.

Whoever becomes president, immigration reform cannot pass without Congress. It needs to craft a sensible and dynamic immigration system.

If the Labor Department determines that a foreign worker — skilled or unskilled — would not displace Americans, that person should not be barred from entering the country by an arbitrary quota. People who want to enter this country to work in jobs Americans are unwilling to take ought to have an easy, legal way to do so.

The raid in Iowa should be a wake-up call.

Ms. Furchtgott-Roth, dfr@hudson.org, former chief economist at the U.S. Department of Labor, is a senior fellow at the Hudson Institute.