NEW YORK (Reuters) - The Trump administration’s proposal to ask a citizenship question on the 2020 U.S. census could lead to an undercount of some 4.2 million among Hispanics, costing their communities federal aid and political representation, according to a new study by Harvard researchers.

People walk with flags as they take part in the annual Hispanic Day Parade in New York, October 12, 2014. REUTERS/Eduardo Munoz

The study by the Harvard Kennedy School’s Shorenstein Center on Media, Politics and Public Policy is the first to assess the impact of the proposed question since U.S. Commerce Secretary Wilbur Ross announced plans last year to reinstate it for the first time in more than half a century.

The study found the question could lead to census-takers missing between 3.9 million and 4.6 million Hispanics nationwide - or between 7.7 percent and 9.1 percent of the Hispanic population recorded in the last U.S. census, in 2010.

Demographers, data experts and even Census Bureau officials have said the question risks frightening immigrants into abstaining from the count in a climate of stepped-up immigration enforcement. Because decennial census data determines how congressional seats are apportioned - and how the U.S. government allocates $800 billion a year in federal aid - an undercount could prove disastrous in some communities.

Harvard said it surveyed some 9,000 people, about half of whom were Hispanic. Researchers found the citizenship question would make all respondents - but especially Hispanics - more likely to skip key questions about race and ethnicity, and less likely to report Hispanic members of their households.

The study may have undersold the impact of the question, the researchers said.

“Not only are we university affiliated academic researchers, and not the U.S. Government... but our respondents were paid panelists and thus financially incentivized to complete the survey,” they said.

The U.S. Commerce Department declined to comment.

Michael Cook, a spokesman for the Census Bureau, said in a statement to Reuters that the bureau is planning to spend $500 million to market the census and promote participation. Cook called the campaign “the most robust marketing and outreach effort in Census history.”

Although a citizenship question has routinely appeared on some Census Bureau surveys, it has not appeared on the mandatory, decennial U.S. census since 1950. Ross says it is needed to help the U.S. Department of Justice better enforce federal protections for minority voters.

Two federal judges have blocked the question for now, siding with Democratic states and cities that alleged Ross’ reasoning was a pretext to repress immigrant participation. The U.S. Supreme Court is slated to hear an appeal on April 23.

The Census, estimated to cost taxpayers $16 billion, becomes more expensive when fewer people respond, as the bureau must pay door-knockers to follow up with noncompliant households.

Ross has come under fire for not testing the citizenship question itself. The Census Bureau is planning to survey the effects of the question in July, to better understand operational needs “such as how many census takers may be needed to follow up and where they should be located,” Cook said.