WASHINGTON (Reuters) - A Republican senator threatened on Tuesday to delay approval of President Barack Obama's choice of ambassador to Brazil because the nominee advocated ending a U.S. tariff on ethanol imports.

"As a senator and as a presidential candidate, President Obama supported keeping the U.S. tariff on imported ethanol," Senator Charles Grassley, who comes from the major corn-producing state of Iowa, said in a statement.

"Now, the president's nominee for ambassador to Brazil says the removal of the tariff would be 'beneficial.' It's important to know whether the administration's position has changed before this nomination goes forward," Grassley said.

Grassley is one of the staunchest defenders of the ethanol tariff in the U.S. Congress. Brazil, the world's biggest exporter of ethanol and its second largest producer after the United States, wants the tariff removed.

Obama nominated Thomas Shannon, a career diplomat who is assistant secretary of state for Western hemisphere affairs, to be U.S. ambassador to Brazil.

The Senate Foreign Relations Committee approved Shannon's nomination on Tuesday by a vote of 14-4, with one abstention. Normally, that approval would set the stage for a full Senate vote.

But Senate rules allow an individual senator to delay floor action on presidential nominations until their concerns are addressed -- or until 60 votes are found in the 100-seat chamber to break the deadlock.

In a letter on Tuesday to U.S. Secretary of State Hillary Clinton and U.S. Trade Representative Ron Kirk, Grassley said comments Shannon made at his July 8 confirmation hearing "calls into question the current position of President Obama on the ethanol tariff."

"Please note that I expect a response to this letter before a confirmation vote on Mr. Shannon's nomination takes place," Grassley said.

(Reporting by Doug Palmer and Susan Cornwell; Editing by Paul Simao)