Donald Trump's vow to round up and deport all of America's undocumented immigrants if he is elected president could shrink the economy by around 2 percent, according to a study to be released on Thursday by conservative think tank the American Action Forum.

The research adds to concerns about the Republican White House nominee's policy proposals, which range from tearing up international trade agreements to building a wall along the U.S. border with Mexico.

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.

Trump, who effectively sealed the Republican nomination this week, has called for the deportation of anyone living in the United States illegally, arguing foreign workers hold down salaries and contribute to unemployment. That position has drawn strong opposition from business leaders like the conservative billionaire Koch brothers as well as from human rights advocates. Trump has further angered opponents by saying Mexico was sending rapists and drug dealers to the United States, and by calling for a temporary ban on Muslims entering the country to shore up national security.

But his hard line stance on immigration has also triggered strong support among many U.S. voters.

Some 52.6 percent of respondents to a Reuters/Ipsos poll conducted in September said they want to see most or all undocumented immigrants deported, compared with 34.6 percent who want to see most or all of them stay.

The American Action Forum analysis used data from the Bureau of Labor Statistics to estimate the value of the output from undocumented immigrants. It also concluded that there would not likely be enough legal employees in these sectors to fill the vacancies that would result from mass deportations.

The study did not factor in potential impacts of mass deportations on consumption, investment and other economic factors, the group said.

The U.S. economy is projected to produce some $18.7 trillion worth of goods and services in 2016, according to the International Monetary Fund. A loss of $400 billion in output would amount to about 2 percent of that figure.