About 6.8 million of the more than 11 million immigrants living in the United States illegally are employed, according to government statistics. Removing them would cause a slump of $381.5 billion to $623.2 billion in private sector output, the Washington-based non-profit said in its analysis.

The study added that removing those workers could leave potentially millions of jobs unfilled due to a lack of legal workers willing to do them. Industries with the highest share of undocumented workers include farming, construction, and hospitality, according to the research.

“The things Donald Trump has said are utterly unworkable,” said Douglas Holtz-Eakin, the forum’s president, and the top economic adviser to Senator John McCain’s 2008 presidential campaign.