Steve Bannon, former White House chief strategist and chairman of Breitbart News, speaks during a discussion on countering violent extremism, at the Ronald Reagan Building and International Trade Center on Oct. 23, 2017 in Washington.Drew Angerer / Getty Images
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Flake, like Sen. Bob Corker of Tennessee, who recently bowed out of his own re-election bid, was facing a brutal campaign against at least one GOP challenger looking to raise hell — and a pile of money from a loosely affiliated network of Trump-backing groups aligned with Bannon, the head of Breitbart News.
Last month, Sen. Luther Strange of Alabama, the favorite of the Republican establishment who had Trump's support, went down in a primary runoff against Bannon-backed Roy Moore.
The Bannonites are becoming the big stick of Trumpism, and Republican establishment elites haven't found effective tools to counter the purge.
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Nick Ayers, chief of staff to Vice President Mike Pence, said as much to top GOP donors at a recent meeting, according to an audio recording obtained by Politico. "Just imagine the possibilities of what can happen if our entire party unifies behind (Trump)," he said. "If — and this sounds crass — we can purge the handful of people who continue to work to defeat him."
And while Pence is much more aligned with the Koch Brothers' network of donors than Bannon's team, those two centers of power have mutual interests in working to advance Trump's agenda.
The immediate aim of Bannon and his allies is to force Republican senators to oust Senate Majority Leader Mitch McConnell, R-Ky. And they're recruiting primary challengers, which by itself can help bring in line some lawmakers who want to keep their jobs. The fear is evident in the decisions by Corker and Flake to not even compete.
If the challengers win Senate seats, they'll be grateful to the Bannon wing — and loyal to Trump. If they lose to Democrats, there will be even fewer McConnell supporters in the Senate.
In short, Bannon & Co. win just by playing.
The establishment wing of the party has little to offer. At one time the national party picked candidates and had a prohibitive lock on the money they needed to win. But in the wake of a series Supreme Court decisions that have allowed for unlimited spending on campaigns, candidates can go around the party by combining the force of grass-roots activism with cash from a handful of wealthy benefactors.
Trump, who poured millions of dollars of his own money into his campaign, rallied a Republican base that wanted to stick it to an establishment that had ignored its concerns.
The conflict began well before Trump, when there were angry exchanges between the base and the establishment and a few high-profile defeats of sitting lawmakers.
But what has changed, according to former Republican National Committee Chairman Michael Steele, is that the base found a way to more consistently turn threats into action.
"What Trump has done is weaponize the relationship between the base and the establishment," Steele said. "Now he's been able to back the threats up by executing on those threats. Bannon is (his) weapon."
It is ironic, Steele said, that some of "the very people being purged" once ran as anti-establishment outsiders. That's true of Corker and of Flake, who was on the conservative fringe as a House member and an ally of Pence on many issues.
But GOP voters are angry that the Trump agenda they endorsed has made little headway in Congress. The Obamacare repeal they were promised died a tortured death this year, and they are looking for retribution. Fairly or not, McConnell has become the face of that failure, and the base's quickest route to retribution is to take out his supporters in the Senate.
For now, Steele said, establishment Republicans are left to wonder: "How the hell did we get into this mess, and how do we get out of it?"
Jonathan Allen is a senior political analyst for NBC News, based in Washington.