Steve Bannon’s declaration of “war” on the GOP establishment has certainly raised eyebrows – and raised hackles within the Republican Party.

Mr. Bannon, President Trump’s former chief strategist, is targeting most incumbent GOP senators up for reelection next year, and recruiting primary challengers in his own (and Mr. Trump’s) populist, nationalist image. His stated goal is to oust Mitch McConnell from his perch as Senate Republican leader, and install a new leader who can pass Trump’s agenda.

But Trump needs success soon on big legislation, like tax reform – so there’s another likely purpose to Bannon’s maneuvering: to be Trump’s secret “whip,” and put pressure on Republican lawmakers who might be tempted to go rogue.

“Trump giving some level of approval to Bannon can actually help him win votes [on legislation],” says Ari Fleischer, former press secretary to the last Republican president, George W. Bush.

Trump softened Bannon’s full-on war cry a bit on Monday. With Senator McConnell at his side in the Rose Garden, the president said he’d try to talk his former aide out of going after the senators he thinks are “great people.”

‘Smart politics,’ but not without risks

To Mr. Fleischer, Trump’s attempt to straddle both worlds – the GOP establishment and the Bannon insurgency – is smart politics. The senators Trump is unhappy with know who they are, and “fear of a primary from the Trump wing of the party, which is a rather sizable wing,” might stop defections on key votes, he says.

But Bannon’s gambit contains considerable risks. On Monday, McConnell rattled off the names of failed GOP Senate nominees who lost winnable races, costing their party the majority – in 2010, Christine O’Donnell of Delaware and Sharron Angle of Nevada, and in 2012, Todd Akin of Missouri and Richard Mourdock of Indiana.

By targeting even safe incumbents, Bannon risks taking sure-thing reelections and making them competitive. But he seems undeterred. “Just voting [with the party] is not good enough, you have to have a sense of urgency,” he said Monday night on Fox News, referring to GOP Sens. John Barrasso of Wyoming and Deb Fischer of Nebraska, both up for reelection in solid red states.

Two GOP Senate incumbents, Jeff Flake of Arizona and Dean Heller of Nevada, entered this midterm cycle already endangered, and now face Bannon-backed primary opponents. Both challengers, Kelli Ward in Arizona and Danny Tarkanian in Nevada, make establishment Republicans uneasy.

In July, Ms. Ward, a former state senator, said that Arizona’s other senator, John McCain (R), should step down after a diagnosis of brain cancer, and suggested she should replace him. Mr. Tarkanian has run for various offices without success. For now, both are polling well among GOP primary voters.

Tea party, take 2

In some ways, Bannon’s “war” is Tea Party 2.0 – with all the risks and excitement that the earlier insurgency brought. Plenty of tea-party candidates won office, including Ted Cruz of Texas – the only Republican senator up for reelection whom Bannon isn’t targeting. (He and Bannon share financial patrons, Robert and Rebekah Mercer.)

But today, the stakes are arguably higher. Trump now faces an outside chance that Republicans could lose their slim 52-48 majority in the Senate, dooming his agenda altogether.

“I still think it’s unlikely” that the GOP will lose control of the Senate, “but a month ago I would have told you it was mathematically impossible,” says Jennifer Duffy, senior editor and Senate-watcher at the nonpartisan Cook Political Report.

With two GOP-held seats already competitive, if just one more goes into the toss-up column, then the Democrats can conceivably take over – assuming they can hold all 23 Democratic seats up this cycle. Again, that’s a tall order, but the Republicans need to be careful, says Ms. Duffy.

“Resources are not infinite,” says Duffy. “Your first job is to protect your incumbents…. Then you go after Democrats.”

Another problem for Bannon and Trump is that most of the renegades in the caucus are not up for reelection in 2018. One who was – Sen. Bob Corker (R) of Tennessee, who has been openly feuding with Trump – opted to retire instead of run again. Sens. McCain, Susan Collins of Maine, Lisa Murkowski of Alaska, and Rand Paul of Kentucky have all bucked Trump on key votes, or threatened to vote no, but none are up this cycle. So it’s not clear that Bannon can intimidate them into voting with Trump on big legislation.

It’s also worth noting that Bannon and the GOP establishment seem to agree on some Senate candidates, such as those running in Tennessee and Missouri. That blurs the lines a bit on the image of a GOP locked in civil war. A report Thursday that Trump had called three senators this week – Sens. Barrasso, Fischer, and Roger Wicker of Mississippi – to reassure them he would support their reelection bids further eased the sense of internecine GOP conflict even as it added to the tension between Trump and Bannon.

Maverick effect?

Then there’s this question: What if Bannon succeeds in electing a group of populist mavericks in the Trump mold? There’s no guarantee they’d be able to replace McConnell with one of their own. And more fundamentally, getting things done in the Senate is all about working constructively with other senators; a bunch of mavericks may not be so inclined.

One key harbinger in all this could be the special Senate election in Alabama on Dec. 12. Former Judge Roy Moore, whom Bannon backed, beat the establishment (and Trump) pick, incumbent Sen. Luther Strange – an outcome that was held up as a sign that the Bannon revolution is on the march. But Judge Moore is controversial, and a recent Fox News poll now shows the race in a dead heat. That poll may be an outlier, but if Democrat Doug Jones actually wins the race in deep-red Alabama, that could be a game-changer.

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Still, to some Republican strategists, Bannon’s war isn’t such a big deal, as he is targeting only a handful of GOP senators.

Only eight Republicans are up for reelection this cycle, and of those, one is not running and three already have firm or likely primary opponents. “It shouldn’t be that hard in this environment for someone of Bannon’s profile to recruit [enough] challengers,” says Constantin Querard, a Republican consultant in Phoenix.