NEW YORK – Top conservative donors admitted at a private gathering on Friday that they may refuse to pony up in 2018 if congressional GOP efforts to overhaul the tax system go the way of healthcare reform.

Mounting frustration and the fear of failure was widespread when some of the nation's wealthiest conservative donors convened in upper Manhattan for a day-long discussion on tax reform and the Senate map in 2018. Beneath crystal chandeliers in the ballroom of the St. Regis, attendees of the Koch-backed seminar received only a tepid guarantee from Republican lawmakers that President Trump will have a tax bill to sign before Christmas.

"Honestly, our entire agenda depends on this Congress stepping forward and delivering on their promise to the American people," Vice President Mike Pence informed the room, highlighting the dire situation congressional Republicans find themselves in after two botched attempts this summer to repeal and replace Obamacare.

Even in the midst of their annoyance with congressional Republicans, most conservative donors were reluctant to entertain the solution proposed by former White House chief strategist Steve Bannon earlier this week.

"We're coming after all of them," the fiery Breitbart News chairman told Fox News on Monday, previewing his plan to challenge all but one Republican in the upcoming midterm election cycle.

Related: Steve Bannon's revolt against Mitch McConnell gets underway

To solve the impasse on Capitol Hill, Bannon has argued that Washington needs fresh blood.

"When you want to talk about why there's no repeal and replace, why there's no tax cut why there's no tax reform, why there's no infrastructure bill, you saw it right there," he said Monday. "[Bob] Corker, [Mitch] McConnell, that entire clique of – establishment clique on Capitol Hill have to go."

"Of course, we're very frustrated," North Carolinian philanthropist and GOP mega-donor Art Pope said Friday, when asked about Bannon's strategy. "But the way to overcome that is not by having a civil war within the Republican Party, but to elect more conservatives and pro-freedom senators."

"So, that is the goal, not to have primaries in some purity contest," added Pope, who was careful to note that he "did intervene to support Marco Rubio" in his primary challenge against then-Republican Gov. Charlie Crist for a Senate seat in 2010.

Denver-based oil magnate Chris Wright said congressional Republicans are likely to "pay a heavy price" next fall if they fail to slash taxes by the end of the year.

But while Wright and his wife, Liz, said they would "absolutely" consider limiting their high-dollar contributions in the midterm cycle, they were less keen on wading into Republican primary season.

"Unlikely, but never say never," Wright said of financially backing insurgent GOP candidates. "Mud-fighting at a local level in politics is not the playing ground for us." Bannon's desire to wage war against incumbent Republicans deepened late last month when Alabama Senate candidate Roy Moore soundly defeated appointed incumbent GOP Sen. Luther Strange in a run-off election. The result left Trump complaining to aides about how he had been convinced to endorse "a loser," a source close to Bannon told the Washington Examiner.

Conservative donors who wish to avoid intra-party primary challenges fear that Bannon, who has full control of Breitbart and maintains a close relationship with billionaire mega-donor Robert Mercer, could do damage in 2018.

The ambitious 63-year-old has been meeting regularly with potential challengers and told Fox News he hopes to challenge every incumbent facing re-election except Texas Sen. Ted Cruz, whose conservatism Bannon's commended and whom Mercer backed in last year's presidential primary.

Amy Walter, national editor of the nonpartisan Cook Political Report, warned conservative donors on Friday of the potential danger Bannon poses to a Republican-controlled Congress if he is able to launch formidable candidates.

"There is no equivalent of Steve Bannon on the Democratic side," Walter said during a panel on the political landscape heading into 2018. "All these moderate [Senate] Democrats – Joe Manchin, Heidi Heitkamp, and Joe Donnelly – they're not getting challenges on the left."

"In 2018, I think Democrats are much more united," Walter told the crowd of conservative donors.