President Trump and former New York mayor Mike Bloomberg, two septuagenarians seeking the White House, traded middle-school-like taunts on Thursday, focused on one candidate’s height and the other’s popularity with his peers.

The exchange offered fresh evidence that Bloomberg, a Democrat who is skipping the first few nominating contests, is more willing than others in the field to engage Trump on his terms and on his favorite playing field: Twitter.

“Mini Mike is a 5’4” mass of dead energy who does not want to be on the debate stage with these professional politicians,” Trump wrote to his 72.4 million followers in a tweet that understated Bloomberg’s reported height by four inches. “No boxes please. He hates Crazy Bernie and will, with enough money, possibly stop him. Bernie’s people will go nuts!”

Trump was referencing Sen. Bernie Sanders (I-Vt.), who has emerged as the leading liberal candidate in the Democratic contest following the Iowa caucuses and the New Hampshire primary.

Shortly afterward, Bloomberg, 77, sought to show that he could return a punch against Trump, 73, who had made his name in the New York real estate world.

“@realDonaldTrump — we know many of the same people in NY,” Bloomberg tweeted to his 2.5 million followers. “Behind your back they laugh at you & call you a carnival barking clown. They know you inherited a fortune & squandered it with stupid deals and incompetence.I have the record & the resources to defeat you. And I will.”

Bloomberg did not directly address an earlier tweet Thursday in which Trump called him “a LOSER” and said his low energy level reminded him of Jeb Bush, a former governor of Florida who was among the Republican White House hopefuls Trump defeated in 2016.