Before President Trump and Steve Bannon's relationship publicly soured, Bannon served as something of a proxy for the president. Long depicted in the press as the philosophical mastermind of Trump's agenda (much to the president's displeasure), he was a favorable figure with obvious sway among Trump's elusive base.
In the Alabama Senate race, for instance, it seemed Bannon and Breitbart's decision to stick with Republican candidate Roy Moore, first in the runoff and then in the general election, made Trump's base more comfortable doing the same, even amid the president's endorsement of Luther Strange and later despite allegations of sexual misconduct leveled against Moore.
My colleague Philip Wegmann raised another good point. Several GOP candidates running in 2018 primaries right now have embraced Bannon as a way of proving their pro-Trump bona fides. Do they disavow him now that he's fallen out of favor with the president?
Bannon has functioned as something of a mascot for Trumpism, both in terms of personality and policy. Neither Bannonism nor Trumpism ever really conformed to any strain of traditional conservatism, but they intersected with each other in important ways. Now, it's their divergences that will become more salient than their intersections.
From my perspective, Trump's base is made up of people with whom his appeal is primarily personality-based or policy-based. Even now, Bannon and Trump's approaches to the personality of governance likely won't diverge, and those who support Trump on the basis of his drain-the-swamp, anti-establishment attitude will probably remain with him so long as that goes unchanged.
But on policy, whether it's immigration or other "globalist" initiatives championed by the likes of Jared Kushner and Ivanka Trump, the segment of the president's base that opposes centrist approaches to these questions could become a key demographic for Bannon to court and exploit going forward. That may spell some trouble for Trump. If Bannon is able to whip loyalists into a frenzy over policy disagreements (consider DACA), sowing discord among his base, that could turn into a serious handicap come 2020.