The U.S. economy could be $1 trillion smaller than otherwise expected in 2021 if Republican candidate Donald Trump wins the presidential election in November, economics research firm Oxford Economics said on Tuesday.

While the firm said Trump's policies - including more protectionist trade measures, tax cuts and mass deportation of illegal immigrants - may be watered down in negotiations with Congress, they could have "adverse" consequences.

"Should Mr. Trump prove more successful in achieving adoption of his policies, the consequences could be far-reaching – knocking 5 percent off the level of U.S. GDP relative to baseline and undermining the anticipated recovery in global growth," it said.

Oxford Economics describes itself as an independent global advisory firm. It is headquartered in Oxford, England, but has offices around the world, including in Chicago, Miami, Philadelphia, San Francisco and Washington.

The Trump campaign did not immediately respond to a request for comment on the research. At a campaign event in Clive, Iowa, on Tuesday, however, Trump reasserted that he would grow the U.S. economy.

He vowed to revive the country's manufacturing sector by preventing U.S. companies such as Apple Inc from making products overseas, renegotiating global trade accords and slashing federal taxes and regulations.

"We’re going to provide opportunity, prosperity and security for all Americans," Trump said.

Under its baseline scenario, Oxford Economics expects U.S. gross domestic product, the value of all goods and services produced in the economy, to grow at a fairly constant rate of around 2 percent from 2017, reaching $18.5 trillion in 2021.

But if Trump is elected and succeeds in implementing his policies, it predicts growth would slow significantly, falling near zero in 2019, and reducing overall GDP to $17.5 trillion.

Oxford Economics said its baseline scenario assumes Trump's Democratic opponent Hillary Clinton triumphs in the Nov. 8 vote and a split Congress emerges - between a Republican U.S. House of Representatives and a Democratic U.S. Senate - which results largely in a continuation of current policies.

Trump would face challenges winning the backing of Congress for all his policies, and some economists argue that looser tax policy could actually help boost economic growth.

The latest opinion polls show Clinton, the former secretary of state, ahead, but her lead has slipped in recent weeks.

(Reporting by John Geddie; Additional reporting by James Oliphant in Des Moines, Iowa; Editing by Howard Goller and Dan Grebler)