Donald Trump has criticised Bubba Wallace and says the NASCAR driver should apologise for what the US President claims is a 'hoax'.
Trump has also attacked NASCAR's decision to ban the Confederate flag at its races and venues.
The President accused Wallace of perpetrating 'a hoax' after one of his crew members discovered a rope shaped like a noose in a garage stall they had been assigned to.
Federal authorities ruled last month that the rope had been hanging there since at least last October and was not a hate crime.
Wallace, who is the only black driver in NASCAR, did not originally find or report the noose, and told NBC: "I was relieved just like many others to know that it wasn't targeted towards me."
But Trump tweeted on Monday: "Has @BubbaWallace apologized to all of those great NASCAR drivers & officials who came to his aid, stood by his side, & were willing to sacrifice everything for him, only to find out that the whole thing was just another HOAX?
"That & Flag decision has caused lowest ratings EVER!"
Wallace has taken an active role in the push for racial equality, racing with a Black Lives Matter paint scheme in Virginia and successfully lobbying for NASCAR's Confederate flag ban.
Following Trump's Twitter post, Wallace published a message to 'the next generation and little ones following my footsteps...'
It read in part: "You will always have people testing you. Seeing if they can knock you off your pedestal.
"I encourage you to keep your head held high and walk proudly on the path you have chosen.
"Always deal with the hate being thrown at you with LOVE! Love over hate every day. Love should come naturally as people are TAUGHT to hate.
"Even when it's HATE from the POTUS (President of the United States)..
"Love wins."
Another driver, Tyler Reddick, has also responded to Trump's tweet.
He said: "We don't need an apology. We did what was right and we will do just fine without your support."
Trump's claim about the NASCAR ratings is also contentious, with reports in America stating that ratings have actually risen of late.