Trump called his rival a “mass of dead energy.” Bloomberg described the president as “a carnival barking clown.”

The two wealthy New Yorkers are longtime rivals. But now that Bloomberg is vying to become the Democratic presidential nominee to face Trump in November, their relationship has turned highly combative.

Trump started the most recent fight early Thursday, calling Bloomberg, whom he calls “Mini Mike,” a “LOSER” in one tweet and a “mass of dead energy” in another.

Bloomberg responded with a warning that showed a familiarity with the president’s past that other Democratic candidates cannot claim. And in the process, he touched on some of the president’s more sensitive subjects: being laughed at behind his back and being called a bad businessman.

Since Bloomberg announced in November that he was running for the Democratic nomination, he has placed most of his focus on attacking Trump instead of on his other primary rivals.

And with no spending restraints, Bloomberg has shown that he can hit the president from different directions with a battery of attacks in television ads airing in key states across the country.

While other Democratic candidates have argued with one another on the debate stages about who is best suited to run against Trump, Bloomberg has focused on criticizing Trump on health care and spent more than $88 million in television ads on the subject since announcing his candidacy.

Trump has focused many of his insults on Bloomberg’s height, which is comparatively much shorter that Trump’s. But the sparring has started to look like they are already running against each other.

Earlier this week, Trump turned to Bloomberg’s record as mayor and called him a “TOTAL RACIST” in a Twitter post in which he shared with his 72.4 million Twitter followers an audio recording of Bloomberg defending a city policy that targeted minorities while he was mayor.

Trump’s tweet was later deleted, and Bloomberg reframed it as the latest example of the president’s “endless efforts to divide Americans.” But before that, Bloomberg had tweeted his defense of the stop-and-frisk policy that Trump had referenced.

The Trump campaign pointed to cable news coverage about Bloomberg’s record on race as a “hugely effective” way to redirect the narrative.

Bloomberg responded that the president had attacked him because of his “fear over the growing strength of my campaign.”

He added, “Make no mistake, Mr. President: I am not afraid of you, and I will not let you bully me or anyone else in America.”

The next day, Bloomberg tweeted, “Donald Trump is the world’s biggest schoolyard bully — with no respect for civility or decency, or facts or honesty.”

This article originally appeared in The New York Times .