Mark Cuban isn't ready to launch a formal campaign to challenge President Donald Trump.

Yet Cuban, an outspoken Texas billionaire who describes himself as "fiercely independent" politically, sees an opportunity for someone to take down the Republican president, who is increasingly viewed as divisive and incompetent even within his own party.

"His base won't turn on him, but if there is someone they can connect to and feel confident in, they might turn away from him," Cuban told The Associated Press. "The door is wide open. It's just a question of who can pull it off."

Indeed, just seven months into the Trump presidency, Republicans and right-leaning independents have begun to contemplate the possibility of an organized bid to take down the sitting president in 2020. It is a herculean task, some say a fantasy: No president in the modern era has been defeated by a member of his own party, and significant political and practical barriers stand in the way.

The Republican National Committee, now run by Trump loyalists, owns the rulebook for nominating the party's standard-bearer and is working with the White House to ensure a process favorable to the president.

Yet Trump's muddled response to a deadly white supremacist rally in Charlottesville, Virginia, this month has emboldened his critics to talk about the once unthinkable.

GOP officials from New Hampshire to Arizona have wondered aloud in recent days about the possibility of a 2020 primary challenge from a fellow Republican or right-leaning independent. No one has stepped forward yet, however, and the list of potential prospects remains small.

Ohio's GOP Gov. John Kasich has not ruled out a second run in 2020. Another Republican and frequent Trump critic, Nebraska Sen. Ben Sasse, last month visited Iowa, which hosts the nation's first presidential caucuses. And a handful of wealthy outsiders including Cuban and wrestler-turned-actor Dwayne "The Rock" Johnson, are being encouraged to join the fray.

Charlottesville response

Trump's comments about Charlottesville "frightened" many Republicans in New Hampshire, said Tom Rath, a veteran Republican strategist in the state that traditionally hosts the nation's first presidential primary election.

"While he has support from his people, the party itself is not married to him," Rath said of his party's president.

Trump denounced bigotry after the Virginia protests, but he also said "very fine people" were on "both sides" of the demonstrations, which drew neo-Nazis, white nationalists and members of the Ku Klux Klan. One woman was killed when a man drove his car into a crowd of counter-protesters.

Even before the divisive remarks, Trump's public approval ratings were bad. Gallup found in mid-August that the president earned the approval of just 34 percent of all adults and 79 percent of Republicans. Both numbers marked personal lows. And as he lashes out at members of his own party with increasing frequency, frustrated Republican officials have raised questions about the first-term president's political future.

On Monday, Maine Sen. Susan Collins said it's "too early to tell" whether Trump would be the GOP presidential nominee in 2020. On Wednesday, Arizona Sen. Jeff Flake said Trump's divisive governing style was "inviting" a primary challenge. And on Thursday night, former Sen. John Danforth, of Missouri, called Trump "the most divisive president in our history" in a Washington Post op-ed.

"There hasn't been a more divisive person in national politics since George Wallace," Danforth wrote.

Trump has also disappointed "The Rock," a former Republican-turned-independent, who told Vanity Fair in May that he'd "like to see a better leadership" from the Republican president.

Trump's response to Charlottesville "felt like a turning point" among those thinking about 2020, said Kenton Tilford, a West Virginia political consultant who founded "Run The Rock 2020." He said the group has already organized volunteers in Iowa and New Hampshire.

"He's vulnerable," Tilford said of the president.

Yet there is good reason why no sitting president since Franklin Pierce in 1852 has been defeated by a member of his own party. As is almost always the case, the most passionate voters in the president's party remain loyal. And in Trump's case, activists across the country are starting to come around.

The president has personally installed his own leadership team at the Republican National Committee and in states like Ohio, Pennsylvania and New Hampshire, where new GOP chairmen are more devout Trump supporters than their predecessors.

As RNC members from across the country gathered in Tennessee this week, leaders had already begun focusing on protecting Trump in 2020.

RNC co-chairman Bob Paduchik, who ran Trump's winning campaign for Ohio last year, was named to lead an RNC effort to review the presidential nominating process in conjunction with White House political advisers.

One possibility, last invoked during President George W. Bush's 2004 re-election, would allow party officials in some states to decide primary contests in closed caucuses without voter input. Such a change could make it all but impossible for another Republican to run a successful nationwide primary challenge.

Two members of the RNC rules committee, Bill Palatucci of New Jersey and Henry Barbour of Mississippi, said they've heard nothing of such an effort.

RNC chairwoman Ronna Romney McDaniel suggested that the blowback for Trump's Charlottesville comments only reminded his hardcore supporters what they like most about him.

"He's not filtered. He's not poll-testing everything. That's part of the appeal he has," McDaniel said. "He has a great understanding of the pulse of the grassroots Republicans right now."

Other RNC members seemed more concerned about the president's statement there were "very fine people" on both sides of the white supremacist rally.

Palatucci said Trump "got it wrong" in his initial comments, but he stands by the president's agenda, especially business deregulation and his recent decision to send more troops to Afghanistan.

Barbour said the confusion following Trump's response to Charlottesville was "a huge distraction." The president's future will brighten, he said, if the GOP-controlled Congress overhauls the tax code and approves sweeping public building projects.

"If he doesn't get those done, we're going to have trouble," Barbour said.

Yet few predicted a significant primary challenge in the most important early voting states.

New Hampshire RNC member Steve Duprey said he's heard no serious talk of one. Said Iowa RNC committeewoman Tamara Scott, "I firmly stand behind my president."