Getting a green card can boost immigrants’ incomes and the number of tax returns filed, according to new research published as a debate over immigration heats up afresh in Washington.

Authors Elizabeth Cascio and Ethan Lewis of Dartmouth College took a look at how receiving permanent U.S. residency affected income tax, through the lens of California and its experience with the Immigration Reform and Control Act of 1986.

Their findings: first, green cards led previously unauthorized residents to file state income tax returns at rates comparable to other Californians. Distributing green cards to about 1.33 million unauthorized immigrants in California led to an increase of around 700,000 state income tax returns annually, Cascio and Lewis found.

Second, while new returns only generated a little extra revenue through the end of the 1990s, “they did raise the earnings of families with children through new claims of the federal Earned Income Tax Credit,” the authors write in the paper circulated by the National Bureau of Economic Research.

The authors said annual EITC transfers to Californians rose, initially by about $500 million, and by $1 billion in the years after the 1994 EITC expansion. The credit’s supporters say it encourages and rewards work.

Green cards are part of an intensifying debate over immigration, a key topic as the November midterm elections approach and government funding is set to expire at the end of September. Over the weekend, President Donald Trump renewed a threat to shut down the government unless Congress approves money for his proposed border wall and enacts new curbs on immigration.

Read:Trump once again threatens government shutdown over immigration.

Trump has called for eliminating what’s known as the green card lottery, which allows more than 50,000 people without family connections or employer sponsorship to enter the U.S. every year. In his State of the Union address in January, he called for a “merit-based immigration system” and reiterated the U.S. should “get rid of” the lottery system in a tweet on Monday.

Opinion:How abolishing the ‘green-card lottery’ would harm U.S. workers.

About a million immigrants receive U.S. green cards each year.