Democratic presidential candidate Kamala Harris said on Saturday endorsements of her by Republican former Vice President Dick Cheney and his daughter Liz, a former US Representative, were "courageous" for putting country ahead of political party.

Harris was in Pittsburgh preparing for the September 10 debate against her Republican rival Donald Trump, with whom she is locked in a tight race for the November 5 vote.

"I'm honoured to have their endorsement," said Harris at Penzeys Spices in the Strip District, on a break from debate preparation where she greeted patrons and bought spices. She said both Cheneys were making a courageous statement that "it's okay, if not important, to put the country above party".

Dick Cheney, who served as vice president under Republican George W Bush from 2001 to 2009, said on Friday that "in our nation's 248-year history, there has never been an individual who is a greater threat to our republic than Donald Trump."

Liz Cheney said on Wednesday she would vote for Harris, calling Trump a "danger."

Trump called Dick Cheney an "irrelevant RINO along with his daughter" in a social media post on Friday, using a term he applies to Republicans not loyal to him, which stands for "Republicans in Name Only." (Reuters)