A union representing U.S. immigration and customs agents urged the Senate on Tuesday to block confirmation of President Donald Trump's nominee to head the immigration enforcement agency, citing past racially tinged and controversial comments.

The National Immigration and Customs Enforcement Council, which represents more than 7,000 agents, endorsed Trump in the 2016 U.S. presidential election. But it opposes the Republican president's nomination of Ronald Vitiello to head the U.S. Immigration and Customs Enforcement (ICE) agency.

A letter from union President Chris Crane to the top Republican and Democrat on the Senate Homeland Security Committee, seen by Reuters, said the nominee's prior offensive tweets demonstrate he "lacks the judgment and professionalism to effectively lead a federal agency."