I think what I did in The Onward March of Progress is pretty much the only way to get him nominated in his own right. Have Nixon, Agnew, and Reagan be disgracefully removed from the scene in the context of domestic calming, peace in Vietnam, and triumphant liberalism, removing conservative candidates and shifting the entire political discourse leftwards. Have 1976 (it basically has to be 1976, to provide enough electoral cycles to cull the conservatives, and to give Brooke time to create connections and a national presence) be considered such a bad election for the Republican party, either due to intraparty infighting, a strong Democratic candidacy, or both, that the A-grade candidates stay out. Have there be just enough contenders so that the race doesn't coalesce into one with a clear frontrunner. Get the other candidates to focus their fire on each other, enabling Brooke to stroll to the nomination through the wreckage. Prevent decisive endorsement deals by having one of the strongest candidates be a hardline conservative in a race that would (given the aforementioned conditions) be dominated by liberals and moderates. And still, it took the left-wing AH.com electorate to get Brooke a mere plurality.