Washington (AFP) - Donald Trump likes to coin disparaging nicknames for his political opponents -- but so far, he hasn't quite hit the mark with rising young Democrat Pete Buttigieg.

Presidential hopefuls "Crazy Bernie" Sanders, "Sleepy Joe" Biden, "Mini-Mike" Bloomberg and "Pocahontas" Elizabeth Warren all crop up frequently in Trump's Twitter feed.

But Buttigieg -- a 38-year-old gay military veteran -- has received surprisingly little presidential abuse despite his rapid emergence as a leading contender for the right to take on Trump in November.

"Bootedgeedge (Buttigieg) is doing pretty well tonight. Giving Crazy Bernie a run for his money. Very interesting!" Trump tweeted after the New Hampshire primary on Tuesday, in which Buttigieg finished a close second to Sanders.

Is it perhaps that Trump doesn't see "Mayor Pete" as a credible opponent? Or is he just struggling to craft a neat epithet for the centrist former mayor of South Bend, Indiana who is reaching out to independents and "future former Republicans"?

For Julian Zelizer, a political history professor at Princeton University, there is no doubt that Buttigieg will soon become a more serious target for Trump if his stellar ascent gathers pace.

"Whenever Trump sees a threat, he attacks the threat. He will be watching and will unleash, should this intensify," Zelizer told AFP.

- What's the angle? -

But what will be his line of attack? Buttigieg's difficult-to-pronounce name?

The president has so far only made passing reference to the tricky surname, pretending to stumble on it when whipping up loyal supporters at rallies.

But the moderate Democrat has already blunted that gambit by joking about it himself, offering voters funny ways to remember how to say it.

Will Trump mock Buttigieg's relative lack of experience? After all, South Bend is a city of 100,000 people, not the world's largest economy.

Buttigieg dismisses criticism of his short political resume by raising his military service in Afghanistan.

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"I've seen a lot worse 'incoming' (fire) than a tweet full of typos," he said, ridiculing Trump's sometimes less-than-stellar spelling on social media.

In a Democratic debate in January, Buttigieg slammed Trump's Vietnam draft deferments for medical reasons.

"I'm ready to take on Donald Trump because when he gets to the tough talk and the chest-thumping, he'll have to stand next to an American war veteran and explain how he pretended bone spurs made him ineligible to serve," Buttigieg said.

What about religion?

Trump has accused Buttigieg of cynically using his Christian faith to boost his election chances. Of course, the Republican leader also wants to win that key bloc of voters.

At a January gathering of evangelicals in Miami, Trump poked fun at Buttigieg's church attendance, saying: "Now all of a sudden, he's become extremely religious, this happened about two weeks ago."

Buttigieg punched back, saying "I'm pretty sure I've been a believer longer than he's been a Republican" -- referring to Trump's past as a Democrat.

- 'Nothing is out of bounds' -

Will he attack the Democrat's sexuality? Buttigieg is the first openly gay candidate with a chance to secure a major political party's presidential nomination.

"Nothing is out of bounds for the president," Zelizer said, but added: "It could provoke a backlash -- though we have seen the president constantly cross these boundaries without any kind of backlash from within the (Republican) party."

Larry Sabato, a political science professor at the University of Virginia, said that if Buttigieg wins the nomination, "the issue of his gay marriage is sure to be front and center. But Trump can let evangelical Christian leaders do his dirty work there."

The lack of a suitable sobriquet for Buttigieg may soon bother Trump, who takes pride in his array of political foes, including his vanished opponent "Crooked Hillary" Clinton.

"I have little nicknames for all of them," he says, but Buttigieg appears hard to nail down.

In one badly failed attempt, the 73-year-old Trump compared Buttigieg last year to Alfred E. Neuman -- the cartoon boy with a wide grin and protruding ears who once graced every cover of humor magazine "Mad."

The likeness is good, but Mad magazine drifted out of public consciousness long ago.

"I'll be honest. I had to Google that," Buttigieg said. "I guess it's just a generational thing. I didn’t get the reference."