Washington: US President Donald Trump has said he is surprised that his Democratic party challenger for the presidential elections Joe Biden has named Indian-origin Senator Kamala Harris as his running mate despite knowing how nasty and disrespectful she has been to him.

Biden on Tuesday named Harris as his Vice-Presidential running mate, making history by selecting the first black woman to compete on a major party's presidential ticket.

Harris, 55, whose father is an African from Jamaica and mother an Indian, is currently the US Senator from California.

We'll see how she works out. She did very, very poorly in the primaries as you know. She was expected to do well. She ended up at right around two per cent, Trump told reporters at a White House news conference.

She had a lot of things happening and so I was a little surprised that he (Biden) picked her. I've been watching her for a long time, and I was a little surprised, Trump said soon after Biden announced that he has picked Harris as his running mate.

Harris had launched her presidential campaign in January 2019. After initial few months, her campaign could not take off and by the end of the year, she withdrew from the race.

She is the first person of colour to be selected on a vice presidential candidate and also only the fourth women and first ever African American and Indian American to be running for the second highest political position in the country.

Responding to questions, Trump said that Harris has been nasty to Biden.

As far as Kamala is concerned, she's a big tax raiser. She's a big slasher of funds for our military. And she's got a lot of difficult things that she's going to have to explain. Plus she was very, very nasty to--one of the reasons it surprised me--she was very--she was probably nastier than even Pocahontas to Joe Biden. She was very disrespectful to Joe Biden, Trump alleged.

It's hard to pick somebody that's that disrespectful when she said things during the debates--during the Democrat primary debates that were horrible about sleepy Joe. And I would think that he wouldn't have picked, Trump said.

Commenting about Vice President Mike Pence, Trump said he has been fantastic at his work.

I will take him over Kamala and the horrible way she, again, treated Justice Brett Kavanaugh. That was a horrible event. I thought it was terrible for her, I thought it was terrible for our nation. I thought she was the meanest, the most horrible, most disrespectful of anybody in the US Senate, Trump said.

Pence said that he was not surprised by this.

"As you all know, Biden and the Democratic Party have been overtaken by the radical left. So given their promises of higher taxes, open borders, socialised medicine, and abortion on demand, it's no surprise that he chose Senator Harris, Pence said at a speech in Arizona.

The vice-presidential debate between him and Harris is on October 7.