WASHINGTON (Reuters) - U.S. President Barack Obama urged Republican presidential nominee Donald Trump to "stop whining" about the Nov. 8 election being rigged, saying no serious person could suggest U.S. elections could be manipulated because of their decentralized nature.

"I have never seen in my lifetime, or in modern political history, any presidential candidate trying to discredit the elections and the election process before votes have even taken place. It's unprecedented," the Democratic president said.

"I'd invite Mr. Trump to stop whining, and go try to make his case to get votes," Obama said at a joint White House news conference with Italian Prime Minister Matteo Renzi.

(Reporting by Ayesha Rascoe, Jeff Mason, Roberta Rampton; Writing by Mohammad Zargham; Editing by Doina Chiacu)