President Trump’s low favorability rating and the ongoing probes into Russian election meddling have ignited speculation that he could be the first sitting president to face a primary challenge since George H.W. Bush beat back conservative firebrand Pat Buchanan in 1992.

Most party strategists say a challenge to the incumbent president from within Trump's orbit is unlikely in 2020.

The blunt-talking commander in chief remains popular among Republican voters, and GOP leaders and conservatives on Capitol Hill have largely stood by their controversial standard bearer.

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Trump’s camp reacted fiercely to a weekend report in The New York Times that said a shadow campaign is emerging in case the 45th president doesn't run. The Times reported Vice President Pence was among those preparing for the possibility, which was strongly denied by the former Indiana governor and White House aides after the story was published.

Still, in this volatile Trump era, few outside the White House would argue that a Republican challenge to Trump is unforeseeable. If the 2020 GOP primary is contested, Trump would be the clear favorite and would have the Republican National Committee behind him. Yet, such a challenge could weaken Trump before the general election — as it did Bush in 1992 before he lost to Bill Clinton William (Bill) Jefferson ClintonDolly Parton remembers Ginsburg: 'Her voice was soft but her message rang loud' Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Calls grow for Biden to expand election map in final sprint MORE.

Here is a look at how the landscape of potential Trump challengers is shaping up, one that could include senators, past presidential rivals and even celebrities.

The senators

Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (R-Ariz.) said Thursday he’s not running for president in 2020, but you could be forgiven for wondering if he’ll change his mind.

The 54-year-old first-term senator has been everywhere — popping up in cable news interviews and penning op-eds — as he promotes his new book, which takes direct aim at Trump and the GOP leaders who Flake says enabled the president’s rise.

“To carry on in the spring of 2017 as if what was happening was anything approaching normalcy required a determined suspension of critical faculties. And tremendous powers of denial,” Flake writes in his book, “Conscience of a Conservative.”

Asked by MSNBC’s Andrea Mitchell if he was eyeing a 2020 White House bid, Flake replied: “No, I'm running for reelection right now.”

Before any possible presidential run, Flake will first have to survive his 2018 race for a second term in the upper chamber.

A new poll found that 62 percent of Arizona voters disapproved of Flake, while only 18 percent approved. And White House officials have been meeting with candidates who are taking on GOP incumbents or mulling a challenge. That includes former GOP state Sen. Kelli Ward, who unsuccessfully challenged Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.) in a primary during the 2016 cycle and has launched another one against Flake.

There isn't much buzz for Flake to run in 2020 — he is a conservative but would be coming at Trump from the left on immigration, a hot-button primary issue.

Another conservative freshman senator, Ben Sasse of Nebraska, is also mentioned by some as a possible Trump opponent.

Regina Thomson and Beau Correll, two former "Never Trump" Republicans who spearheaded a movement to allow delegates at the Republican National Convention to vote freely instead of according to primary and caucus results, told The Hill separately that Sasse, 45, is the quintessential conservative that could pose a real threat to Trump in a primary.

“Sasse has maintained both his credibility and dignity prior to Trump's election and afterward,” Correll said. “He calls it as he sees it, which will be highly regarded amongst well-reasoned Republican primary voters.”

But in an email, Sasse spokesman James Wegmann called these 2020 primary stories “bonkers.”

“It's 2017 and Ben has the only callings he wants: raising his three kids and serving Nebraskans,” Wegmann said.

Meanwhile, Sen. Tom Cotton Tom Bryant CottonRenewed focus on Trump's Supreme Court list after Ginsburg's death Republicans call for DOJ to prosecute Netflix executives for releasing 'Cuties' Loeffler calls for hearing in wake of Netflix's 'Cuties' MORE (R-Ark.) has positioned himself nicely as an ally of the administration with foreign policy credentials and the respect of the base. However, Cotton is only 40 years old and running against Trump could hurt his political future. He's also up for reelection in 2020.

“Senator Cotton is focused on serving the people of Arkansas and advancing the president’s agenda of growing the economy and opportunities for Arkansans [by] lowering taxes, repealing and replacing Obamacare, and fixing our broken immigration system,” said Cotton spokeswoman Caroline Rabbitt.

The centrists

Ohio Gov. John Kasich was the last GOP challenger to Trump in 2016, not because of his primary success, but because everyone else read the writing on the wall.

Republicans say they don’t know what Kasich is planning, but they’re sure he’s up to something.

That could mean an independent run, potentially on a bipartisan ticket with a fellow centrist, like Colorado Gov. John Hickenlooper, a Democrat.

But no one would be surprised to see Kasich challenge Trump in the primary again.

Kasich, who has been on a book tour, will be out of the governor’s mansion in January 2019 and seems eager to tout his version of conservatism as an alternative to Trump’s.

Still, many Republicans would view a primary challenge as an ego-fueled vanity project. As a moderate with views on immigration and healthcare that are anathema to the base, Kasich is not a good match for the GOP primary electorate. He won only one state in the 2016 primary: his home state of Ohio.

“All this makes for interesting cocktail chatter, even with the waterboarding of Pence and little Nicky in the White House basement,” said Kasich’s political strategist, John Weaver, referencing the blowback Pence and his chief of staff Nick Ayers received in response to the Times story on the vice president’s political ambitions.

“Governor Kasich is only focused on being the very best executive for the people of Ohio and on policy issues like healthcare, national security, trade, economic expansion and ending the growing gap between the haves and have-nots.”

House conservatives

Most of Rep. Justin Amash Justin AmashOn The Trail: How Nancy Pelosi could improbably become president History is on Edward Snowden's side: Now it's time to give him a full pardon Trump says he's considering Snowden pardon MORE’s attacks against the president come in a venue Trump knows well: Twitter.

The Michigan Republican, a co-founder of the conservative House Freedom Caucus and one of the nation’s most well-known libertarians, has mocked Trump’s understanding of the Constitution, questioned whether the president’s global investments and projects pose conflicts of interest and signed on to legislation calling for an independent probe into Russian election meddling.

Amash, who backed Sen. Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (R-Ky.) in last year’s GOP primary, also was the first Republican in Congress to raise the possibility of impeaching Trump. If it’s determined that Trump pressured his then-FBI director, James Comey, to end the bureau's Russia-related investigation into Trump campaign associates, Amash said in May, then that would be grounds for impeachment.

National Republicans are skeptical, noting that Amash lacks name recognition and a national fundraising network and hails from the libertarian wing of the party, which has historically underperformed in GOP primaries. Through his spokeswoman, Amash had no comment.

The celebrities

Trump’s shocking rise — bolstered by his celebrity and near-universal name recognition — has greatly expanded the field of candidates that political operatives are likely to view as legitimate contenders.

While many Republicans might believe it’s safer for their political futures to remain on the sidelines rather than get in the ring with Trump, a well-funded outsider could see an opening to shake things up in the GOP primary rather than take the third-party or independent path, which is viewed by most experts as hopeless.

On this front, the Democrats have a more robust bench, but there are a handful of celebrities with mixed political backgrounds that could consider a GOP run if they think Trump is vulnerable.

Dallas Mavericks owner Mark Cuban has relished sparring with Trump. Cuban, who has become more recognizable from the hit TV show “Shark Tank,” has liberal social views but has said he is otherwise more in line with Republicans.

"I have been contacted by people from both parties, although not by the national organizations,” Cuban told The Hill in an email. “It's something I am considering but am not ready to make a decision on.”

Actor and former professional wrestler Dwayne "The Rock" Johnson is awash in social media buzz and has publicly expressed his interest in running. Johnson is a registered independent but has attended the national conventions for both Democrats and Republicans.

Another wealthy independent, former New York City Mayor Michael Bloomberg, will generate speculation until he definitively rules out the possibility of running. The billionaire founder and CEO of the Bloomberg financial services and media company has been both a Republican and a Democrat. Bloomberg, 75, indicated earlier this year that his political aspirations are over: "I’ve got plenty of things to do. And maybe I’ll run for president of my block association, but not much more than that."

Waiting in the wings

GOP strategists and insiders interviewed by The Hill remain skeptical that anyone will enter a primary challenge against Trump, describing the endeavor as a “suicide mission.”

But a raft of high-profile Republicans are believed to be keeping their fingers on the pulse of the terrain so that they’re ready in case the 71-year-old president is politically crippled by the Russia investigations, loses his base of support or simply determines that he’s happier as a private businessman. However, people in Trump's orbit have said the president will run again.

If there is a Trump-sized void, Pence would instantly become the front-runner. Pence, 58, has conservative bona fides and the Trump campaign has essentially never ended, so the vice president continues to bank fundraising and email equity.

Florida Gov. Rick Scott (R) has been making moves behind the scenes in preparation for national office. Trump has publicly called on Scott to run against Sen. Bill Nelson Clarence (Bill) William NelsonDemocrats sound alarm on possible election chaos Trump, facing trouble in Florida, goes all in NASA names DC headquarters after agency's first Black female engineer Mary W. Jackson MORE (D-Fla.) in 2018.

And then there are the 2016 runners-up: Sens. Ted Cruz Rafael (Ted) Edward CruzVideo of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Democrat on Graham video urging people to 'use my words against me': 'Done' MORE (Texas), Marco Rubio Marco Antonio RubioSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Florida senators pushing to keep Daylight Savings Time during pandemic Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll MORE (Fla.) and Paul, as well as Wisconsin Gov. Scott Walker, all have appeal among sections of the GOP primary electorate and could be eager for redemption, even if a unilateral challenge to Trump is unlikely.