Bannon by himself isn't that powerful, but Mercer $$$ can go along way. We really shouldn't underestimate the potency of this project of his.
Indeed we shouldn't. Bannon may not be that powerful himself, but the primary candidates he's endorsing are the types that appeal to anti-establishment Republicans that are motivated primary voters. Establishment GOP voters certainly won't be the ones driving primary election enthusiasm given the string of failures of full-Republican government control, but pissed off tea partiers and Trumpists will be very motivated to show up to the polls in the primary to foist their preferred lunatics on the party to send a message to Congressional leadership.