Republican strategists and activists increasingly fear that a failure to deliver on tax reform in the coming months will intensify primary challenges to sitting incumbents next year and imperil the party's already precarious standing in the midterm elections.

Angry GOP donors, a restless conservative base, a standstill Congress and a uniquely impetuous president are raising the stakes for a fourth-quarter legislative agenda that will be largely defined by an attempt at revamping the tax code that has languished for months.

An outside insurrection by Breitbart News head and former White House chief strategist Steve Bannon already is ominously fanning the flames of internecine warfare. But many top Republican minds believe the most powerful tipping point for the GOP is whether it can deliver on Trump's key campaign promise of producing tax relief for Americans.

"If Congress passes the key elements of the conservative agenda, including repealing Obamacare and cutting taxes, some of the anger at the grass roots will dissipate," says Ralph Reed, founder and chairman of the Faith & Freedom Coalition. "But if Congress fails to do so, I think there will be a lot of primaries in 2018 and 2020, and I think there will be a lot of vulnerable incumbents."

Saddled by multiple failed attempts to repeal former President Barack Obama's health care law, President Donald Trump and congressional Republicans are now turning their concerted attention to pitching lower tax rates and a simplification of the filing system. But there's a growing realization they are now up against a calendar that leaves only two and a half months until an election year – and some of the most fiery activists already have lost their patience.

The latest evidence of intraparty unrest came Wednesday in the form of a blistering letter from leading conservative groups asking Senate Majority Leader Mitch McConnell and members of his leadership team to step aside, citing their failure to act on an array of issues from illegal immigration and deficit spending to Planned Parenthood funding and a repeal of the Affordable Care Act.

"Republicans were given full control of the federal government. They – you – have done nothing," the letter reads. "Worse, it is painfully clear that you intend to do nothing because, as is most apparent, you had no intention of honoring your solemn commitments to the American people. You were not going to drain the swamp. You are the swamp."

The searing missive was signed by Ken Cuccinelli, president of the Senate Conservatives Fund; Jenny Beth Martin, co-founder of Tea Party Patriots; Adam Brandon, president of FreedomWorks; David Bozell, president of ForAmerica; Brent Bozell, chairman of ForAmerica; and conservative activist Richard Viguerie.

The cadre also questioned McConnell's "commitment to real reform" on taxes – and a key GOP member of the House Ways and Means Committee on Wednesday acknowledged lawmakers will have to settle for at least some changes that won't be permanent. "We're not going to do as well as we had hoped in terms of permanence. It's obvious," said Rep. Pete Roskam of Illinois.

Meanwhile, even as Bannon's clarion call for primary challengers to half a dozen GOP Senate incumbents has shaken the political media establishment as he intended, many GOP campaign veterans privately contend his influence has been widely overblown.

Plenty of anti-establishment candidates and would-be contenders mulling 2018 bids were stirring the pot long before Bannon came along. Alabama's Roy Moore, for example, was beating Sen. Luther Strange ahead of Bannon's blessing. Arizona's Kelli Ward had run in 2016 against Sen. John McCain, and shortly after that defeat switched her focus to Sen. Jeff Flake.

Mississippi's Chris McDaniel, who is inching closer to a challenge of GOP Sen. Roger Wicker, gained national notoriety in 2014 for falling barely short in his bid to unseat Sen. Thad Cochran.

Bannon is also in talks with potential challengers to Sen. John Barrasso in Wyoming and Sen. Orrin Hatch in Utah, but so far neither has drawn a formal primary opponent, and Hatch hasn't even formally decided to run again. In Nebraska, one key GOP player mocked any Bannon effort to draft a candidate to run against first-term Sen. Deb Fischer. "There's really not any anti-Deb sentiment in Nebraska," says Mike Kennedy, a 25-year GOP activist from Omaha. "I don't see any traction for Bannon at all. They're going to have to look under a lot of rocks."

"Let's be honest: Steve's a drum major desperately running in front of a parade," says a prominent conservative activist, speaking anonymously because he counts Bannon as a friend. "He's good copy. He's a good story. The issue is not Bannon. The issue is what these people were told for eight years: That when we got the White House, the Senate and the House, this stuff was going to happen. The grass roots feel like they've been played."

"If we don't pass the tax cut, I think all bets are off," the activist adds, referring to the number of ferocious primaries that could multiply across the map.

Strategists working to preserve and expand the 52-member Republican Senate majority are also pinning their hopes on tax reform to hand their incumbents a tangible accomplishment that will land in voters' pocketbooks. At the same time, they know it stands to impact their own bottom lines.

A Senate GOP source acknowledges fundraising has begun to lag since June and that the National Republican Senatorial Committee – the entity tasked with electing GOP senators – has spent more than it's raised over the preceding two months.

"Donors are so pissed off," the source says. "If we don't get tax reform, we won't have the money to fund all our races. They just don't understand why nothing's been done."

Terry Schilling, executive director of conservative think tank the American Principles Project, agrees that Republicans need an accomplishment on tax reform that they can hold in front of voters next year.

But unlike others, he doesn't view Bannon's efforts as necessarily counterproductive. Instead, Schilling says, Bannon's looming threat of outside fire provides a constant incentive for even the most dependable incumbents to make good on Trump's agenda.