<p>Halfway through Wednesday’s off-the-rails CNBC debate, Republican National Committee Chairman Reince Priebus realized he was witnessing a chain-reaction wreck. By the time it ended, he knew it was headed straight for him. </p><p>Priebus has always been keenly aware that the Republican Party has become increasingly vulnerable to outside forces, starting with the unexpected emergence this year of uncontrollable to-hell-with-the-establishment candidates like Donald Trump and Ben Carson.