In a letter to President Trump and top Congressional leaders Wednesday, nearly 1,500 economists extolled the economic benefits immigrants bring to the U.S. and urged Congress to "modernize" the country's immigration system.

"[I]mmigration is one of America's significant competitive advantages in the global economy," the letter said. "With the proper and necessary safeguards in place, immigration represents an opportunity rather than a threat to our economy and to American workers."

Among the economic benefits that immigration brings are entrepreneurs who start businesses, young workers who replace retiring Baby Boomers and people with diverse skill sets to keep American companies competitive and innovative in high-growth fields like STEM, the letter stated.

The letter was signed by economists from across the political spectrum, including Douglas Holtz-Eakin, who served under President George H.W. Bush, and Austan Goolsbee, the former chairman of President Obama's Council of Economic Advisers. The group also included six Nobel laureates.

Related: Trump's merit-based immigration system: Who would get in?

"Immigration is not just a good thing," said Eakin, who is now president of the right leaning American Action Forum think tank. "It's a necessity."

Last month, a survey of 285 economists at America's major corporations by the National Association of Business Economics (NABE) found that a clear majority believed President Trump's restrictive stance on immigration is a mistake. These economists favored more "relaxed immigration policies" that they said would help boost the economy and noted that fixing the H-1B visa program should be the priority over deporting illegal immigrants.

Earlier this year, Trump said he wanted to move away from a family-based immigration system toward a merit-based one, like Canada's or Australia's, which gives preference to immigrants with education and skills. However, merit-based systems also typically allow for some family reunification and refugees.

Eakin said immigration reform should include family reunification, as well as visas for both highly skilled and so-called low-skilled workers.

Related: Low-skilled immigration is petering out, even without a wall

"They don't all have to be STEM workers," he said. "Markets reward skills that have value and this day and age, running a machine tool is a skilled trade. You should have a visa system that recognizes market shortages and allows those skills to enter the United States."

The letter acknowledged that immigration does come at a cost, particularly for Americans with lower levels of education. But it said the benefits -- especially with smart immigration reform in place -- far outweigh the costs.

"We urge Congress to modernize our immigration system in a way that maximizes the opportunity immigration can bring, and reaffirms continuing the rich history of welcoming immigrants to the United States," the letter said.

The letter was organized by the Partnership for a New American Economy, a coalition of business leaders and mayors launched by former New York Mayor Michael Bloomberg and media mogul Rupert Murdoch, and the American Action Forum.