

Steve Bannon is seen in Harrisburg, Pa., on April 29. (Carolyn Kaster/AP)

It's difficult to overstate how scary this paragraph, from Bloomberg's Jennifer Jacobs and Bill Allison, should be for the Republican Party establishment:

Steve Bannon plans to back primary challengers to almost every Republican senator who runs for reelection next year in an effort to depose Majority Leader Mitch McConnell and streamline Senate voting procedures, three people familiar with his plans said.

Breitbart's Matt Boyle, who obviously has some intel on this, dutifully provides some pro-Bannon spin and the names of potential primary challengers here. (The one GOP senator reportedly getting a pass? Ted Cruz.) And now The Post's Robert Costa is adding one: GOP megadonor Foster Friess, who is weighing a primary challenge to Sen. John Barrasso (R-Wyo.) after talking to Bannon in recent days.

More than anything, Bannon seems to be wielding a threat: He wants McConnell out and the filibuster nuked, or else. But it also seems highly unlikely the GOP and McConnell will give him what he wants anytime soon. And that means Bannon will be put in the position of making good on these threats.

We've already seen what that can produce, with Roy Moore's virulently anti-McConnell, Bannon-backed campaign emerging victorious in Alabama. Exactly how much credit Bannon deserves for that is an open question — Moore was leading before Bannon came onboard — but the anti-McConnell message sure seemed to resonate in that race. Bannon is threatening to take that national.

And as importantly, Bannon's advocacy could help these GOP challengers clear a threshold that most of them fail to ever meet. The major hurdle to defeating Republican incumbents in recent years hasn't been in knocking them down; it's been in finding a candidate who is well-funded enough to actually make it a competitive campaign in the first place. Incumbent senators spend years banking millions of dollars, and challengers often can't even get off the ground — or don't even try. There's never been a hugely organized national effort to support these challengers, and generally only one or two a cycle even clear the threshold.

But when they do, it's usually competitive. To wit:

In 2016, former state senator Kelli Ward came within 13 points of Sen. John McCain (R-Ariz.) after raising $1.5 million

In 2014, state Sen. Chris McDaniel fell to Sen. Thad Cochran (R-Miss.) by half a point after raising $2.6 million

Also in 2014, Dr. Milton Wolf fell to Sen. Pat Roberts (R-Kan.) by seven points with $1.3 million behind his candidacy

No incumbent senators lost either year, but every well-funded challenge made things reasonably close — as far as incumbent primaries go. And you needn't go back too far to find incumbents who did fall. Sen. Richard Lugar (R-Ind.) lost his primary in 2012. Two years prior, both Sens. Bob Bennett (R-Utah) and Lisa Murkowski (R-Alaska) lost to tea party primary challengers, and Sen. Arlen Specter (Pa.) switched to the Democratic Party in the face of his own possible defeat.

And the soil for primary challengers is looking fertile this year. The limited polling we have suggests both Sens. Dean Heller (R-Nev.) and Jeff Flake (R-Ariz.), for example, are highly vulnerable to their primary foes despite facing candidates who have run and failed before. Their numbers are pretty brutal in fact, with several polls showing Flake trailing repeat candidate Ward by doubt-digits and Heller running behind perennial candidate Danny Tarkanian in at least one poll.

Both candidates have seen their numbers plummet, apparently thanks in part to running afoul of President Trump. And so has McConnell. Given that admittedly sparse data, it's not difficult to see an environment forming in which other Republicans could see their jobs at risk. And if Bannon and wealthy donors like the Breitbart-backing Mercer family and Friess can get these candidates off the ground, that's a scary prospect. Even defeating one or two of them would send a message to otherwise-safe GOP senators that they had better toe the Bannon line — or risk their own ousters.

The good news for Republicans is that, outside of Heller and Flake, most every incumbent is running in a clearly red state where the party would be favored to hold the seat even if a Bannon-backed candidate prevails. But that doesn't mean this won't be painful.