Donald Trump 's name-calling is apparently getting aggressive in private ...

'cause a new report claims he's calling his presidential opponent a brutal name behind close doors. The New York Times published a piece today diving behind the scenes of the Trump campaign over the last few weeks ..

. and, in it, the Times details what they call the "foul mood" Trump's been in over the last few weeks. Among other alleged angry actions and remarks .

.. NYT says two anonymous sources claim he's called Kamala Harris a "bitch" multiple times -- and, he's reportedly attacking her at donor events too.

NYT reporters provide details about his fundraising dinner in the Hamptons -- and, they say some of the attendees were also shocked about Trump doubling down on recent comments about Harris' race. Remember ..

. while speaking to the National Association of Black Journalists, Trump claimed Harris only recently began identifying as Black -- comments that landed him in hot water last week. NYT says he told attendees he still thinks he's right despite all the backlash he's receiving .

.. before apparently going on a rant about KH.

Now, Trump reportedly using a swear word to attack his opponent isn't unique ...

remember, back in February reports came out that Joe Biden regularly refers to Trump as a "sick f***." Video evidence seems to show him nearly doing it at a rally. Worth noting .

.. Trump spokesman Steven Cheung told the Times DJT has never called Harris a "bitch" -- adding the campaign wouldn'.