Students change classrooms during a break at International Academy of T.C. Williams High School in Alexandria in 2014. The International Academy is a high school program for immigrant children who arrive with little or no English skills. (Astrid Riecken/For The Washington Post)

Tomás R. Jiménez is an associate sociology professor at Stanford University and a faculty affiliate at Stanford’s Center for Comparative Studies in Race and Ethnicity.

After the White House abandoned its policy of separating children from their migrant parents, Congress had a little breathing room to try to pass immigration legislation addressing other significant issues. But Republican hard-liners in the House who have foiled past immigration reform attempts have been at it again. In recent days they forced postponement of votes on compromise legislation, and on Wednesday led a revolt against House leadership in voting down the bill.

There is an irony here: Anti-immigration lawmakers and their supporters frequently cite the failure of illegal immigrants to assimilate as a reason for their hostility, yet they fight the very kind of reforms that encourages assimilation. The best way to foster the integration of undocumented immigrants into American society is to provide a path to legal status. The conservative hero Ronald Reagan knew that legalization works, and he acted on it.

In 1986, Reagan signed the Immigration Reform and Control Act, which included an amnesty for roughly 2.7 million undocumented immigrants. Reagan got it right in his signing statement when he said: "The legalization provisions in this act will go far to improve the lives of a class of individuals who now must hide in the shadows, without access to many of the benefits of a free and open society." Emerging from the shadows and enjoying the fruits of American society is assimilation in action.

The federal government has generally left it to immigrants and the society they encounter to work out the terms of assimilation without any policy intervention. And that has worked pretty well. Assimilation among today's mostly Latin American and Asian immigrant groups looks a lot like it did generations ago, when most immigrants came from Europe. That's the conclusion reached in 2015 by a panel of experts assembled by the National Academies of Sciences, Engineering and Medicine after an exhaustive review of the research. The panel noted that across the generations, today's immigrant groups are catching up to and even exceeding the general U.S. population in a host of important social and economic indicators. They're speaking English, too, with virtually all children of immigrants fluent by adulthood. This assimilation happens despite little to no policy support.

So, what's the problem? Legal status. There is no greater threat to assimilation than the undocumented status of the roughly 11 million residents in the United States illegally. Two-thirds of these immigrants have been in the country for more than a decade, so a degree of assimilation is inevitable: 64 percent are employed, half speak English and almost a third own homes, according to the Migration Policy Institute. But their undocumented status prevents them from further integrating into American society.

Reagan understood how to address the problem, and his signing statement proved prophetic. Sociologists Frank Bean, Susan Brown and James Bachmeier studied the effects of the amnesty on Mexican immigrants and subsequent U.S.-born generations in the Los Angeles area. Legalization was a deciding factor in their assimilation, the authors found in 2015. Compared with Mexican immigrants who didn't legalize, those who did went on to acquire more education and to fare better in the labor force.

But the real story, the sociologists discovered, was the effect of legalization generations later. The grandchildren of those who gained legal status gained far more education and better-paying jobs. Granddaughters of immigrants who legalized fared especially well, reaching socioeconomic parity with U.S.-born white women.

The closest thing we have to the 1986 legalization is Deferred Action for Childhood Arrivals, known as DACA, an Obama-era executive order that provides a reprieve from deportation for certain undocumented immigrants. Unlike the 1986 legislation, DACA doesn't offer a pathway to citizenship. But even DACA's limited protection has made it a de facto assimilation program. Political scientist Tom Wong's survey of 3,000 DACA recipients finds that of those who qualified for DACA, 7 in 10 said they got a better job; more than half found jobs more commensurate with their training and career goals; and roughly the same proportion took jobs with better working conditions. Among school-enrolled DACA recipients, nearly three-quarters were working toward a bachelor's or advanced degree.

DACA is also good for the next generation. I'm part of a team of social scientists led by Stanford University's Immigration Policy Lab that examined mental health indicators among the U.S.-born, citizen children of undocumented parents in Oregon. We found that DACA eligibility made a big difference: U.S.-born children of DACA-eligible mothers had better mental-health indicators compared with children whose mothers were undocumented and ineligible for DACA.

Despite DACA's positive effect as an assimilation driver, President Trump rescinded the policy last fall, and anti-immigrant Republicans in Congress balk at reform legislation that might restore it. Meanwhile, the nation's 11 million undocumented immigrants stay in limbo as the last traces of Ronald Reagan's brand of welcoming conservatism become ever harder to detect among rising Republican nativism.