Heeding the Obama administration’s call for immigration reform, a bipartisan group of eight senators Monday released a proposal they plan to introduce as legislation. They wisely included legalization for current undocumented immigrants, but their plan will likely come up short on a guest-worker program for legal migrant workers.

While legalization is a good step, lack of a comprehensive guest-worker program only perpetuates the problem many immigration critics cite as their biggest concern: unauthorized immigration. Yet guest-worker measures have worked in this country before, so it is pure politics, rather than substance, that prevents officials from crafting one now.

Unauthorized immigrants who are not violent or criminals should indeed be legalized. They came here for economic security, and many are on their way to achieving it. So many of their offspring, the so-called “DREAMers,” who were brought here as children, know nothing but the United States and speak only English. They are Americans in all the ways that count — except a law that now says they aren’t. It’s time for the law to accept them.

Most unauthorized immigrants came from Mexico, Central America and Asia, where the benefits of moving here are incomes two to six times greater than in their homelands. The U.S. economy, struggling since the 2008 crisis, put a damper on unauthorized immigration, but gradually recovering housing and labor markets are beginning to attract people again.

That is why a robust guest-worker program is needed: to accommodate future flows of migrants. After decades of unauthorized immigration motivated by economic gain, it is fantasy to expect it to stop after legalizing those unauthorized immigrants already here. Let us not forget that President Ronald Reagan tried an amnesty in 1986 — which failed because it legalized the workers here but did not provide a viable pathway for future workers to come.

What we need is a legal way for lower-skilled immigrants to enter the United States — and a guest-worker visa program is the easiest avenue.

So why doesn’t the proposed immigration reform include a comprehensive guest-worker program? Surprisingly, the main issue is not opposition from conservative Republicans. It is unions and their supporters who do not want it.

In the 2007 immigration reform push, an amendment that would have ended the guest-worker program after five years destroyed Republican support.

The then-leaders of the AFL-CIO, the Laborers’ International Union of North America, the United Food and Commercial Workers International Union, the International Brotherhood of Boilermakers, the Iron Ship Builders, Blacksmiths, Forgers and Helpers and the Teamsters all wrote letters opposing guest workers and supporting the amendment.

Teamsters President James P. Hoffa wrote that he opposed a guest-worker program because it would “[force] workers to toil in a truly temporary status with a high risk of exploitation and abuse by those seeking cheap labor.”

At least unions now support immigration restrictions for different reasons than before. Samuel Gompers, the founder of the AFL, never saw an anti-immigration bill he did not like. He supported the Chinese Exclusion Act and the near-race-based quota acts of the 1920s that drastically curtailed immigration from anywhere except Northern and Western Europe.

Gompers’ logic was simple. He thought unions can only organize when there is a limited supply of labor, especially immigrant labor. This thinking was even espoused by the likes of César Chavez, whose organizing efforts involved calling the Immigration and Naturalization Service to report unauthorized Mexican workers or forming a “wet line” (his language) on the Mexican border.

Now unions say they oppose a guest-worker visa program to protect these workers from abuse. But unauthorized workers are going to come in any case, so preventing a guest-worker program can only place them in a black market — where employer abuse, backed up by the threat of deportation, is far worse.

If unions are honestly concerned about guest-worker abuse, the solution is making the visa portable and not tied to one employer. A World War I guest-worker visa let guest workers quit their jobs and be hired by approved employers. Guest workers simply had to tell the government about their new employer after they were hired — not seek permission before switching jobs.

The best labor protection is a worker’s ability to quit a job without legal sanction. If the government could create such a guest-worker visa program 100 years ago, there is no reason why it cannot be revived today.

As long as there is economic opportunity here, immigrants — legal or not — will come. An immigration bill that does not create a vehicle for legal migrants to enter the country is not real reform.