The bishop of Brooklyn in New York is defending undocumented workers in the U.S. and has decried the mass deportation proposal of presumptive Republican presidential nominee Donald Trump.

Writing in The Tablet, the newspaper of the Diocese of Brooklyn, Bishop Nicholas DiMarzio actually did not name Trump although he's the only candidate who is espousing the idea.

"Those who are in favor of mass deportation seem not to have an understanding of what this might mean for our reputation, and the lives of those who are deported. The estimated costs of mass deportation would be $400 billion, and [it would] reduce the Gross Domestic Product (GDP) by about $1 trillion," Bishop DiMarzio wrote.

DiMarzio said the issue of undocumented workers in the U.S. is unnecessarily being politicised, adding that the problem can be resolved without getting into racist and xenophobic tendencies.

Citing research, he said honest workers deserved to be defended as they contributed to the society and economy and because they are human beings with dignity, rights and responsibilities.

"The issue of illegality is one that is exploited and misunderstood," he said.

"Undocumented immigrants have either entered without inspection or, more commonly in recent years, have over-stayed or otherwise violated the terms of their temporary visas by working with false documents. As human beings, they cannot be '"illegal,'" DiMarzio said.

According to the chief actuary of the Social Security Administration, unauthorised workers make an annual contribution of $12 billion to Social Security although most of them will never collect Social Security benefits themselves, the bishop said.

The Institute on Taxation and Economic Policy estimated that undocumented immigrants pay $11.6 billion per year in income, sales and property taxes.

DiMarzio said data from the Journal on Migration and Human Security indicate that the undocumented population in the U.S. has been falling in the last eight years.

He said the main problem is the path to citizenship.

"To exclude the undocumented from citizenship would be to return to a two-tiered society. We have enough experience with the exclusion of former slaves and their descendants to remind us that those who are members in our society should never be excluded from the full rights of citizenship," he said.

DiMarzio said Americans should always remember that their nation was built by immigrants. "We cannot forget the contributions of the past nor the present made by new Americans in building our society and our Church," he said.