CLEVELAND -- The wife of the presumptive U.S. presidential nominee Donald Trump headlined the first day of the Republican National Convention with a rare public speech that resoundingly endorsed her husband.

"Donald is, and always has been, an amazing leader," a somewhat nervous-looking Melania Trump said Monday. "Now, he will go to work for you."

Donald Trump appeared briefly to introduce his wife -- "an amazing mother, an incredible woman" -- in a move unusual for a presidential candidate, since such convention debuts are normally reserved for nomination night. The real estate tycoon promised cheering supporters that "we are going to win, we are going to win so big," his statements punctuated by the Queen rock anthem "We Are the Champions."

Melania Trump touched on her background as a Slovenian immigrant, calling U.S. citizenship "the greatest privilege on planet Earth." In her husband, the one-time supermodel said she sees a man who "will never, ever, give up."

"And, most importantly, he will never, ever let you down," she added.

American media outlets reported Monday that parts of the speech appeared to have been lifted from an address by Michelle Obama, now first lady, at the 2008 Democratic National Convention. The Trump campaign denies allegations of plagiarism.

Besides English and French, Melania Trump is fluent in three other languages. A win by her husband would make her the first foreign-born first lady since John Quincy Adams' wife nearly two centuries ago. The 46-year-old mother usually does not participate in her husband's campaign, choosing instead to spend time with their son. She has told an interviewer that she hopes to be a traditional first lady.