Some 18.6 million illegal immigrants now call the U.S. their home, according to a new study by an immigration control group that says the numbers soared under former President Joseph R. Biden.

The Federation for American Immigration Reform said the figure stood at 14.5 million in late 2020, meaning a rise of 4.1 million people over the four years of Mr. Biden’s tenure.

“Americans have seen the impact of the illegal immigration crisis play out in their communities all across the country. This new report provides hard data on the enormity of the illegal immigration surge that took place as a result of policies put in place by the Biden administration,” FAIR President Dan Stein said.

He said President Trump is taking steps to reduce the inflow, but he said the report shows how much work the new administration has to do in rousting those already here.

FAIR’s estimate is at the top end of guesses about the total illegal immigrant population.

The organization uses Census Bureau data, then adjusts them based on its own estimates for how much the bureau is undercounting illegal immigrants. It assumes a larger undercount than other studies — though FAIR says recent trends and the Census Bureau’s own admissions give credence to its more aggressive estimates.

FAIR’s calculation includes people Mr. Biden allowed to enter the country under legally iffy “parole” programs or who are here under temporary deportation amnesties such as Deferred Action for Childhood Arrivals. While they may have some current “lawful presence,” they do not have a legal visa to be here.

The Center for Migration Studies, based in New York, in its latest estimate released last year, calculated the illegal immigrant population at 11.7 million as of July 2023.

The Center for Immigration Studies, based in Washington, put the figure last year at 14 million, though it said that could be an undercount.