Trump lost to Cruz in the 2016 primary. Both Trump and Cruz are Kobach Republicans.
...and carried the state by smaller margins than either McCain...
No. McCain only won Kansas by 15.4 points. Trump wont by 20.4 points.
or Romney did.
Romney won by 22.2 point or 1.8 points more than Trump but he didn't have to deal with a strong libertarian candidate.
A curious thing that Trump and Brownback shook loose a lot of so-called “Moderates” (sic) infesting the KS GOP. Since the Demonrats have been historically a very weak party in the state, there were effectively two parties within the GOP. The regular Conservatives and the “Moderate” (hard-left) faction.
The “Moderates” would be Demonrats in almost any other state. Most stayed in the GOP for reasons of power, but in the past decade, they’ve been starting to finally exit the party and become the Demonrats they already are. As Brownback was the first Conservative GOP Governor in nearly 50 years, that was bad enough for them. With the rise of Trump (and Kobach), that sent many of them over the edge.
As with working against Kobach, they had similarly conspired in 2002 to stop another Conservative, Tim Shallenburger, who had won the GOP Gubernatorial nomination and backed the ultraleft Dem Kathleen Gilligan Sebelius (daughter of a Socialist Dem Governor of OH who was so bad that he managed to lose reelection to a Republican in the Watergate year of 1974). So Kobach was not the first victim of RINO backstabbers.