People who weren’t even born in America are taking significant advantage of one of Obamacare’s single-most expensive benefits, the expansion of Medicaid, a new report from the Center for Immigration Studies (CIS) shows. In fact, immigrants have accounted for 42 percent of the growth in Medicaid enrollment since Obamacare began being implemented in 2011, the report finds.

“The high rate and significant growth in Medicaid associated with immigrants is mainly the result of a legal immigration system that admits large numbers of immigrants with relatively low-levels of education, many of whom end up poor and uninsured,” report co-author Steven Camarota of CIS says. “This fact, coupled with the extensive supports we provide to low-income residents unavoidably creates very significant costs for taxpayers.”

The report, which Camarota authored with Karen Zeigler, finds that from 2011 to 2013 a whopping 42 percent of Obamacare’s Medicaid expansion went to immigrants and their children rather than to native-born U.S. citizens and their families.

Camarota and Zeigler wrote that it “seems almost certain” that such immigrants and their children “will continue to benefit disproportionately from Obamacare, as they remain much more likely than natives to be uninsured or poor.”

The report finds that the immigrants who are taking advantage of these government benefits are generally those who immigrated to the U.S. legally. That means illegal aliens, who could be granted executive amnesty by President Obama if he moves forward with his plans for such an order, would also likely have access to the Obamacare benefits.

Incoming Senate Budget Committee chairman Jeff Sessions (R-AL) is planning to launch a battle using government funding mechanisms to block President Obama from implementing the executive order. But even as Sessions and other GOP leaders plan to block funding for Obama’s plans, some Republicans from the establishment side of the party, including House Appropriations Committee chairman Rep. Hal Rogers (R-KY), are seeking to push an omnibus spending bill that could include funding for Obama’s planned amnesty and fund the entire government until the end of the 2015 fiscal year in September.

Sessions and his side of the fight argue that Republicans, who just won big in the midterm elections and took the U.S. Senate majority, would be in effect surrendering the mandate voters gave them to lead the nation to outgoing Senate Majority Leader Harry Reid should they do what Rogers and other establishment Republicans are pushing.

If Rogers and his side of the party succeed, it’s all but certain that Obama’s planned executive amnesty will happen—and the staggering numbers of immigrants getting access to Obamacare’s Medicaid expansion, which helps the unpopular law survive, would likely increase.

For now, however, CIS found that immigrants and their U.S.-born children are gaining access to the Obamacare provision twice as often as American natives. Immigrants have seen an 11 percent increase in Medicaid enrollment under Obamacare, while the rest of the population has seen a 5 percent increase.

The Medicaid increase among immigrants due to Obamacare, the report found, is costing U.S. taxpayers about $4.6 billion per year, and the report found this increase in immigrant Medicaid access is actually helping them gain more access to healthcare than American natives’ access.

“Partly because of increased Medicaid enrollment, the share of immigrants and their children without health insurance declined more dramatically than for natives, from 28 percent in 2011 to 23 percent in 2013 — a five percentage-point decline,” the report found. “Among natives and their children, it fell from 13 percent to 11 percent — a two percentage-point decline.”

The data for the CIS comes from the Current Population Survey’s Annual Social and Economic Supplements and other government sources.