Washington: President Donald Trump has been quick to cook up derisive nicknames for his Democratic critics and rivals: "Crooked" Hillary Clinton, "Goofy" Elizabeth Warren and "Crazy" Bernie Sanders, among others.

Now one of them has returned the favor. Former vice president Joe Biden, hitting the campaign trail Friday for the first time since joining a crowded Democratic field of presidential aspirants, has dubbed Trump a "clown."

Biden — or "Sleepy Joe" as Trump calls him — offered the new nickname in response to a question raised at a small fundraising event in Columbia, South Carolina, according to a pool report and a readout from the Biden campaign.

A supporter in the room asked Biden how he would respond to Trump's often insulting language, saying, "I want you to fight back; once he throws a nickname at you, you gotta throw a nickname back."

Biden drew laughter when he replied: "There's so many nicknames I'm inclined to give this guy. You can just start with clown."

Biden, a veteran of decades of political wars, said he would never shy from responding to Trump's attacks, but did not want to let the president use them to turn attention from serious issues. "On every single issue and on every demeaning thing he says about other people, I have no problem responding directly," Biden said.

"What I'm not going to do is get into what he wants me to do. He wants this to be a mud-wrestling match." Trump, no youngster at 72, has already made an issue of Biden's greater age (the former Obama vice president is 76).

"I'm the youngest person, I am a young, vibrant man," a smiling Trump told reporters at the White House on April 27. He added, "I look at Joe, I don't know about him." Trump might have a harder time drawing that contrast against others in the Democratic field, several of whom are younger than 50.