RUTHFULLY YOURS

With the Senate beginning debate this week on the immigration reform bill, border security will be one of the most contentious issues. Sen. Marco Rubio (R., Fla.) argues at every opportunity that “immigration reform hinges on border security,” and he isn’t wrong. But physical control of the border can go only so far in preventing illegal immigration. At least as important is finding a way for immigrants to work here legally. That’s where a guest-worker program is crucial.

The U.S. workforce is changing. Americans are having smaller families, and birthrates are well below replacement level. Baby boomers are retiring: 10,000 leave the workforce every day. Younger workers coming up behind them are much more educated than earlier generations. In 1950, according to the Census Bureau, 56% of U.S. workers were high-school dropouts. Today, the figure is less than 5%.

The result is that the pool of people available to fill low-skilled jobs has shrunk dramatically. It is not so much that the native born don’t want to work as busboys, farmhands or nurse’s aides. But the overwhelming majority of Americans are now overqualified for these jobs and have other options. Meanwhile, less-skilled immigrants with no family in the U.S. have no way—no access to a visa program—to enter the country legally and work in year-round jobs. This is why so many immigrants have flowed into the country illegally in recent decades and remain here, underground.

The challenge facing Congress is to create a better system—one that works for willing immigrant workers and willing employers, replaces the current illegal influx with a legal labor force, and protects the rights of Americans who are looking for low-skilled jobs.

The worker-visa program in the Senate bill meets most of these tests. Participating employers must first try to hire American workers, recruiting widely and offering a decent wage. If they can’t, the program gives them a way to hire foreign workers easily, quickly and legally.

The work visas in the Senate bill bear little resemblance to traditional guest-worker programs, which generally tie the worker to one employer, opening the door to exploitation and an inefficient labor market. Instead, under the Senate proposal, an immigrant here on a work visa can quit any time and go to work for any other U.S. employer that has been approved to participate in the program. Business owners get access to a more flexible labor force, including the possibility of hiring in real time without going back to the government for approval.

The size of the guest-worker program is designed to adjust automatically in response to changing U.S. labor needs, growing in good years when the economy needs more foreign workers and shrinking when more Americans are out of work.

Finally, unlike most guest-worker programs, the Senate proposal recognizes that some participants will want to stay in the U.S. when their visas have expired. Those who have done well while on temporary visas—learning English, developing new skills, moving up on the job, putting down roots in their community—may apply to stay permanently. This is a win-win for the immigrants and for the U.S., combining the benefits of a temporary worker program with those of the nation’s traditional way of immigration—the melting-pot model based on permanent residence and assimilation.

The one area where the Senate proposal falls short: It’s almost surely too small to address the country’s future labor needs. In the early 2000s, when the economy was booming, more than 350,000 unauthorized Mexicans entered the U.S. every year to fill low-skilled jobs for which there were not enough Americans. The downturn eased this demand somewhat, but it’s now picking up again.

If the trends of recent decades resume, the Senate proposal won’t be big enough to divert future illegal immigration. In its first year, the Senate’s guest-worker program will be capped at 20,000 visas. It will grow over time in response to market demand but is capped at 200,000 workers annually. The danger is that if there aren’t enough visas to meet U.S. labor needs, the program won’t prevent illegal immigration in years ahead.

The best antidote to illegal immigration is a legal immigration system that works. The Senate’s visa program is a good start. The challenge now, for the House or the Senate, is to scale up the Senate model so it can work in years to come.

Ms. Jacoby is a fellow at the New America Foundation and president of ImmigrationWorks USA, a national federation of small business owners working for better immigration law.

A version of this article appeared June 12, 2013, on page A17 in the U.S. edition of The Wall Street Journal, with the headline: Guest Workers Are the Best Border Security.