Trump's 2016 playbook vanquished Republican challengers. Can he do it again in 2024?

When Donald Trump ran for president in 2016, he faced a Republican primary field of seasoned politicians and leaders. He went on to beat them all, often by deploying visceral personal attacks -- from "little Marco" and "low energy Jeb" to "lyin' Ted," whose father Trump once baselessly suggested was linked to the assassination of John F. Kennedy.

But now, as he wages a third consecutive White House campaign, the power of Trump's punches is likely to be put to its greatest test, according to veterans of his scorched-earth style.

Some of them predict that if he faces a new slew of GOP challengers, which would be a highly unusual obstacle for a former president running in a primary, he'll successfully chew them all up, in a repeat of his 2016 playbook.

Or he will choke trying, other party operatives said, regardless of the damage it may do to the broader party.

"Remember 2016: 16 of our best Republican candidates walked into a dark alley with Donald Trump and only one person emerged -- and it was Trump," said one of his 2016 campaign staffers, who was granted anonymity to speak candidly about the former president's comeback attempt.

"He's a Rottweiler," this former staffer said. "He will eviscerate whatever's in front of him."

So far, the only person Trump has had to deal with on the 2024 campaign is himself.

Since launching his bid on Nov. 15, he's been dogged by new controversies -- dining with the antisemite Nick Fuentes, saying the Constitution should be partially terminated -- and ongoing legal scrutiny as well as a mixed track record with his political power, after his candidates in marquee midterm races went down in defeat.

In that same time, Trump has done little if any actual campaigning, beyond statements on social media, while he staffs up for a run that will likely begin in earnest in the coming months.

Speculation over whether an early launch by Trump would clear the primary field has been replaced instead by guesses over just how many opponents he might face. Trump remains very popular among Republican voters, polling shows, but other figures could eclipse him: An early-December poll from The Wall Street Journal found Florida Gov. Ron DeSantis beating Trump in a hypothetical head-to-head primary contest, 52-38%. However, Trump is unlikely to just have one opponent and any nomination fight is still a year away.

"It hasn't been going as well as any of us had hoped," admitted another Trump 2016 campaign staffer, who remains in touch with the former president's team.

The first staffer, though, said there was reason for optimism once Trump was no longer running alone.

"He's had a tough couple of weeks because every boxer who shadowboxes by themselves gets tired," they said. "You put somebody in the ring with him, I think we know how this ends."

A spokesman for Trump declined to comment on the record for this story.

'A very feral intelligence'

If Trump has to run against DeSantis in the 2024 Republican primary -- or former Vice President Mike Pence, former Arkansas Gov. Asa Hutchinson or others -- it won't be the first time he's faced such competition.

As soon as he launched his 2016 campaign in June 2015, Trump broke from other Republicans by voicing views on policy and people that were considered outside the GOP's tent or were outright insulting, as when he said some Mexican migrants were "good people" and some were "rapists."

He quickly found himself in the most crowded GOP primary in decades, alongside former Florida Gov. Jeb Bush, former Ohio Gov. John Kasich, Texas Sen. Ted Cruz, Florida Sen. Marco Rubio and more.

Trump relied on his decades of celebrity and touted his business acumen rather than any time spent in the "swamp" of national politics. He bulldozed his competition with a mix of unorthodox populist policies, which had largely been absent from contemporary conservatism, and ceaseless attacks.

"Look at that face! Would anyone vote for that?" Trump once said on the trail of competitor Carly Fiorina, a former technology executive.

"He was out there hunting in the field, and he'd find the next person and take them out. He has a very feral intelligence at doing that," said David Kochel, an adviser to Bush's 2016 campaign.

"Even though Trump has had … what I think a lot of people think is a pretty weak start, once you got a bunch of people on the field, he'll kick into action," Kochel said. "It's kind of like 'RoboCop.' As soon as there are candidates in the race, he's gonna come to life and start shooting at them."

Already, Trump has been road-testing attack lines. At a rally in November, he labeled DeSantis as "Ron DeSanctimonious" -- and later derided him as an "average" governor who benefited from "great Public Relations." Trump also insisted his endorsement for DeSantis in 2018 helped pave DeSantis' path to Florida's governorship. At the time, two DeSantis sources told ABC News that the DeSantis team made a decision not to respond.

A candidate's appetite for such onslaughts must be part of the decision over whether to challenge him, those who helped campaign against Trump said.

"It could boost him a lot," Alex Conant, a top staffer to Rubio's 2016 campaign, said of Trump's penchant for insults. "He's very good at finding somebody's weakness and exploiting it. And so any candidate needs to be eyes wide open that Trump is merciless and be ready to fight."

Some Trump allies crow that his style can be politically lethal.

"All you have to do is look at 2016. Where's Jeb Bush? Where's John Kasich? The list goes on," said the campaign staffer still in touch with Trump's team. Meanwhile other former rivals, like Cruz and Rubio, became ardent supporters.

In a big primary field, Trump's enduring base of support -- which some strategists estimated to be about 30% of GOP voters -- could also help push him over the finish line. In 2016, for example, he won the Republican nomination with about 45% of the votes; the majority was mostly split between Cruz, Kasich and Rubio.

But do Trump's hits still hurt?

Trump's GOP critics say he is a figure diminished -- damaged first by his 2020 election loss as well as nearly nonstop controversy, the defeats of his handpicked midterm candidates and the enduring cloud from Jan. 6 and other investigations.

In this view, he still holds political sway but not absolute power in his party.

Two months after declaring his 2024 campaign, "The desired effect of clearing the field was not achieved," said one GOP strategist advising a potential primary rival to Trump. "In fact ... it's probably inspired a few other people that may have been on the fence, may be unsure, but now they're like, ‘OK, well, that wasn't much.'"

This strategist had seen Trump as "the 600-pound gorilla, not quite the 800-pound gorilla. I'd say he's losing weight fast."

Trump is also likely to face off against several candidates who served in prominent roles in his administration and even more who are broadly supportive of the policies he pursued, while a potential rival like DeSantis could hammer differences with Trump over COVID-19, given Trump's role in initial restrictions and vaccine development.

"When you turn to basically say, 'I want to finish the [border] wall. I want to get tough on China. I want to bring back American energy independence and lower gas prices, I want to invest in the military,' or whatever they're talking about, he may try to go attacking them individually. But from a policy standpoint, it would be very hard to do," said one longtime party strategist.

As divided as the GOP may be over Trump's standing, Republicans who spoke with ABC News are similarly torn over how candidates can best respond to Trump's incoming fire.

The adviser to a potential Trump opponent advocated for ignoring the attacks, suggesting the novelty has worn off.

"I sense that there are a lot of people in the party that are kind of over that," they said. "It was entertaining and it was fun and it was different, and we all like to sit back and watch the food fight in the cafeteria. But after about the third or fourth food fight, you're like, 'Man, I just want to eat my lunch.'"

Others, however, said that Trump's sharp tongue can't be ignored.

"They [the base] want a fighter. But the challenge is going to be how you thread the needle of fighting for the issues, for the people, fighting to defend your positions and not directly fighting him," said the longtime GOP strategist said.

Striking that balance is "gonna be very difficult," this person said. "Because at the end of the day, and this is true of any primary, [the pitch is] 'even though we're gonna fight it out in a primary, I need your support in the general.'"