Former South Carolina Gov. Nikki Haley, who served as ambassador to the United Nations during former President Donald Trump’s administration, is expected to declare her candidacy for president Wednesday, joining Trump as the only major Republicans to date to have launched White House campaigns.

Haley spotlighted her countdown on Monday, tweeting, "Just two days until we see you all in South Carolina."

So far, it has remained a field of one in the hunt for the GOP presidential nomination since Trump jumped into the race in mid-November. But Haley’s anticipated entry — at an event in Charleston, South Carolina — could open the flood gates, with other likely Republican White House hopefuls launching their own campaigns in the weeks and months ahead.

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The big question is: How large a field will materialize?

With Trump already in and with Florida Gov. Ron DeSantis – who’s national brand has soared among conservatives the past couple of years – potentially launching a bid later this year – campaign professionals aren’t expecting a very large field. Especially compared to the 17 Republicans who ran in the 2016 presidential cycle or the whopping 27 Democrats who ran in 2020, which were wide-open races for their respective parties.

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

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Longtime GOP consultant David Kochel told Fox News that "it’s probably going to be a smaller field than in 2016."

"I think it’s going to be much more manageable. I’d say we’re going to end up with six to eight candidates," Kochel, a veteran of numerous presidential and Iowa based campaigns, predicted.

And longtime Republican pollster Neil Newhouse, who’s also pulled multiple tours of duty on GOP presidential campaigns, emphasized that "it’s going to be a smaller field of competitors than it certainly was eight years ago."

One key reason is Trump – who more than two years after his 2020 reelection defeat at the hands of President Joe Biden – remains the most popular and influential politician in the GOP, and he is arguably the party’s top fundraiser when it comes to energizing the grassroots.

"People are looking at this race in [assessing] Trump’s strength, and all the numbers indicate he’s still pulling a significant percentage of Republicans, making it really difficult for somebody to break through," Newhouse said. "It’s going to be difficult for an unknown to catch fire in this election simply because they’re running against a guy who’s extraordinarily well known and liked by many in the party. I think that’s going to limit the field. People are going to self-select whether they can do this or not."

"He is significantly stronger at this point in time in the 2024 election than he was in the 2016 election," Newhouse said of Trump. And pointing to the open race for the 2016 GOP nomination, he added that "it’s a little more daunting right now running against a former incumbent president than it was eight years ago."

But the first three months of Trump’s latest White House bid have raised plenty of questions regarding his political durability.

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Pundits from both the left and the right criticized his campaign launch as well as controversial actions and comments he’s made since declaring his candidacy. And in the wake of a lackluster performance by the GOP in the midterm elections – when the party underperformed in what many expected to be a red wave election – Trump has also been blamed for elevating polarizing Republican nominees who ended up losing in November.

While the former president was once the overall front-runner in the early 2024 GOP nomination polls, DeSantis has eclipsed him in some surveys over the past few months. Nearly every poll indicates Trump and DeSantis as the favorites, with everyone else in the single digits.

DeSantis, a former congressman, saw his popularity soar among conservatives across the country the past three years, courtesy of his forceful pushback against coronavirus pandemic restrictions and his aggressive actions as a conservative culture warrior, going after media and corporations. And the Florida governor's nearly 20-point reelection victory helped transform the one-time blockbuster battleground into a red state.

DeSantis routinely dismisses talk of a 2024 White House run, but he’s dropped plenty of hints of a possible presidential bid since his reelection victory speech in November.

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Sources in DeSantis’ wider orbit say any presidential campaign launch wouldn’t occur until late spring or early summer, after the end of Florida’s current legislative session. But Republican sources confirm to Fox News that the governor’s political team has already started reaching out and identifying operatives for a potential White House run.

Tom Rath, a longtime GOP consultant and former New Hampshire attorney general, said the White House race is "a battle for oxygen and attention" and that both Trump and DeSantis are now taking up nearly all the "oxygen and attention."

He said that "it’s hard for some of the others to emerge."

Kochel said that Trump and DeSantis – if the Florida governor launches a campaign – would be in a first tier "getting the most attention," with a second tier of candidates angling for opportunities.

That second tier would likely include former Vice President Mike Pence, who on Wednesday will be in Iowa, the state whose caucuses kick off the Republican presidential nominating calendar.

Sen. Tim Scott of South Carolina, whom pundits view as a potential presidential contender, heads to Iowa the following week as part of a listening tour that was first reported recently by Fox News.

Among the others making moves toward launching a campaign or seriously considering a Republican presidential run are former Secretary of State Mike Pompeo, who’s currently on a book tour; now-former Govs. Larry Hogan of Maryland and Asa Hutchinson of Arkansas; Govs. Chris Sununu of New Hampshire, Kristi Noem of South Dakota and Glenn Youngkin of Virginia; former Gov. Chris Christie of New Jersey and former Rep. Will Hurd of Texas.

Kochel predicted that "they’re definitely people snipping around that won’t run or might just test the waters and not pull the trigger."

But Merrill forecast that Republicans will "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."