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.