Donald Trump Jr. claims Barack Obama "plagiarized" seven words from his speech at the Republican National Convention, during the president's address to Democrats on Wednesday.

The son of the Republican presidential nominee claimed on Twitter that Obama used his line, "That is not the America I know," during Obama's Democratic National Convention appearance.

"I'm honoured that POTUS would plagiarize a line from my speech last week," Trump wrote. "Where's the outrage?"

Trump said the words while talking about the changes his father will bring in, if he is elected.

"There's so much work to do," Trump Jr. said in his speech. "We will not accept the current state our country because it's too hard to change. That's not the America I know."

Obama evoked the phrase in his speech, to describe the elder Donald Trump's RNC address.

"What we heard was a deeply pessimistic vision of a country where we turn against each other, and turn away from the rest of the world," he said. "There were no serious solutions to pressing problems – just the fanning of resentment, and blame, and anger, and hate. And that is not the America I know."

But if Trump Jr. is hoping to lay claim to that particular sequence of seven words, he doesn't have much of a case. Former president George W. Bush used the same words in a 2002 speech on the Iraq war, and Obama used the words in a 2010 speech in Cleveland.

Trump also said "that's," not "that is," if we want to split hairs on the issue.

The accusation comes after Donald Trump Jr.'s stepmother, Melania Trump, was caught repurposing passages from a 2008 speech by Michelle Obama in her own speech at the RNC. Melania Trump faced widespread criticism for reusing the first lady's words, although her speechwriter later took the blame for the gaffe.

"A person she has always liked is Michelle Obama," Trump speechwriter Meredith McIver said, in a statement issued last week. "Over the phone, she read me some passages from Mrs. Obama's speech as examples. I wrote them down and later included some of the phrasing in the draft that ultimately became the final speech."

Melania Trump's speech included a reworked story originally told by Michelle Obama, about learning the value of hard work and honesty as a child.

Melania Trump echoed Obama in speaking about the values she learned as a child. Melania Trump said: "From a young age, my parents impressed on me the values that you work hard for what you want in life, that your word is your bond and you do what you say and keep your promise, that you treat people with respect. They taught and showed me values and morals in their daily life."

Michelle Obama said, in 2008: "And Barack and I were raised with so many of the same values: like, you work hard for what you want in life, that your word is your bond, that you do what you say you're going to do, that you treat people with dignity and respect, even if you don't know them and even if you don't agree with them."