Four years ago this month, the 2020 Democratic presidential nomination race was in full swing.

Sens. Bernie Sanders of Vermont, Amy Klobuchar of Minnesota and Cory Booker of New Jersey were days away from declaring their candidacies.

Sen. Kamala Harris was announcing her campaign and then-South Bend, Indiana, Mayor Pete Buttigieg was launching an exploratory committee, which Sens. Elizabeth Warren of Massachusetts and Kirsten Gillibrand of New York had already done days earlier.

Then-Rep. Tulsi Gabbard of Hawaii and former Housing Secretary Julian Castro were declared candidates, and long shots like Andrew Yang and former Rep. John Delaney had already been in the race for well over a year.

Fast-forward four years, and it’s a very different story in the 2024 race for the Republican presidential nomination.

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The only declared candidate is former President Donald Trump, whose campaign has been mostly flying under the radar since his launch at his Mar-a-Lago club in Palm Beach, Florida in mid-November.

"It certainly does seem slower this time," longtime GOP consultant David Kochel noted.

A number of well-known Republicans are seriously mulling a 2024 White House run, with some of them making the early moves that come ahead of a formal campaign launch. That list includes Florida Gov. Ron DeSantis, former Vice President Mike Pence, former Secretary of State and former CIA director Mike Pompeo, former South Carolina Gov. and Ambassador to the United Nations Nikki Haley, now-former Govs. Larry Hogan of Maryland and Asa Hutchison of Arkansas, Sens. Ted Cruz of Texas, Tim Scott of South Carolina, and Rick Scott of Florida, Govs. Chris Sununu of New Hampshire and Kristi Noem of South Dakota, former Gov. Chris Christie of New Jersey, and former Rep. Will Hurd of Texas.

But to date, none of these likely or potential contenders has come close to actually launching a campaign.

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And one main reason is the former president. 

More than two years after his 2020 election defeat at the hands of President Biden, Trump remains the most influential politician and ferocious fundraiser in the Republican Party, and until recently, he was the clear and overwhelming front-runner in the early 2024 GOP presidential nomination polls.

"You have an incumbent to some degree in Donald Trump. As much he would be a challenger in the general election, you still have to look at him as an incumbent in the primaries," a longtime Republican consultant and veteran of numerous presidential campaigns told Fox News.

"The real question is who wants to be second to Trump," added the consultant, who asked for anonymity to speak more freely. "I don’t think Pence or DeSantis – who appear to be the two of the big heavyweights out there – I don’t think either of them want to be the second one in."

Kochel, a veteran of numerous GOP presidential campaigns in Iowa and nationally, emphasized that once someone else jumps into the race, "Trump’s going to turn on the guns and come after you and start to draw contrasts and find the right nickname."

"There’s no benefit for someone like DeSantis, who will have all the money he needs, to get in early and start the burn rate. There’s no upside to it," Kochel argued. But he stressed that for some of the lesser-known candidates, "the incentive would be to get in, try to say some things that really make news and drive attention. Strategically I think there’s reasons for some of these lesser-known candidates to get going soon."

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Another reason for the slow start may be name recognition.

"Many of the people who are considering this – DeSantis, Pence, Pompeo, Haley – they’re pretty much universal names" among Republican primary voters, the GOP consultant said. "I don’t think they have as much of a need to jump in this as quickly as opposed to some of the people who are known in their states but not well-known across the country."

Trump's 2024 campaign launch didn't grab rave reviews, and it was criticized not only by Democrats, but also by fellow Republicans. Some in Trump's political orbit told Fox News the early announcement was intended in part to clear the field of potential rivals and help the former president avoid the growing net of legal entanglements, but it appears to have failed on both accounts.

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Trump also appears to be the victim of self-inflicted wounds from his heavily criticized dinner at Mar-a-Lago over the Thanksgiving holiday with the antisemitic rapper Ye – formerly known as Kanye West – and White nationalist and Holocaust denier Nick Fuentes, to a widely panned social media post which appeared to suggest the "termination" of the U.S. Constitution, as well as a profitable but mocked roll out of digital trading cards portraying Trump as a superhero, and his controversial abortion comments earlier this month that received pushback from some social conservatives in the party’s base.

Trump has also faced plenty of incoming fire over the midterm election losses of GOP nominees handpicked and supported by the former president, which were a contributing factor in the lackluster results for Republicans in November in what many had hoped would be a red wave year.

These stumbles may be another contributing factor in apparent hesitancy of other potential contenders from launching campaigns. 

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"I do feel they think the longer Trump is by himself, it maybe gives the others an advantage, because there’s more of a focus on Trump's negatives rather than his positives," the consultant who asked to remain anonymous argued. "I do think there’s some tendency to say, ‘Let’s let Trump implode.’ Nobody feels like Trump’s getting a lot of momentum."

Kochel pointed out that "Trump is best when he’s counter punching, when he’s drawing contrasts. So, denying him the direct conflict is also a way to starve him of oxygen."

The conventional wisdom is that once a potential GOP presidential contender launches a campaign, the flood gates will open and others will follow.

But no one’s expecting a very large field – similar to the 17 Republicans who ran in the 2016 presidential cycle, or the 27 Democrats who ran in 2020, which were wide open races for their respective parties.

New Hampshire-based Republican strategist Jim Merrill, a veteran of numerous GOP presidential campaigns, predicted with Trump in the race, "instead of 12-16 candidates, you might end up getting 6-10."

He added, "You’re going to have a robust field. I don’t think he’s [Trump] clearing anybody out. The losses the Republican Party has taken the last three cycles make it clear that people are going to be clamoring for different voices. I think we need to have a competitive primary and I think you’re going to get one."

But longtime GOP consultant and former New Hampshire attorney general Tom Rath noted that the White House race is "a battle for oxygen and attention, and Trump takes up so much oxygen and attention, it’s hard for some of the others to emerge."

And Rath, who’s also a presidential campaign veteran, argued that "the coverage seems to have anointed DeSantis as the alternative."