What has been the worst decision/action on the GOP primary side this year?

[Attorney General, Senator-Elect, & Former PPT Dwarven Dragon]

Write-In: Rather than leaping in behind the actual establishment favorite, John Kasich, who earned the title by placing second in NH, the establishment money forces catapulting themselves behind "Republican Obama (Rubio)" for racial reasons and perhaps an ideological quirk or two with Kasich. Once they realized Rubio was going nowhere, they had left Kasich on fumes for so long that he was utterly useless outside of Ohio (and Manhattan).

Beyond that, the major candidates either ignoring TRUMP or forming bromances with him until January for Cruz and February for the rest was really stupid. Throw CruzCarly in there as well.

[Attorney General, Senator-Elect, & Former PPT Dwarven Dragon]

Write-In: Allowing Chris Christie onto the debate stage on February 6th

Without that, Marco Rubio places 2nd in NH, completes the 3-2-1 strategy in SC, and has a cakewalk to the nomination.

Maybe not the whole cakewalk thing. The fact is that Rubio was never going to win in some of those southern ST contests (and yes, Cruz would have stayed in the race.). So you basically get a more even three way contest. And this is assuming Rubio doesn't make some drastic mistake in one of the later debates. Remember - one slight change in history can really alter things in extremely weird ways.