An alleged discussion on nukes attracted new criticism for Trump: Trump asks why US can't use nukes.
In the 2008 debates, Romney, Giuliani, and Duncan Hunter all stated that using tactical nukes was an option. Duncan Hunter even endorsed preemptive nuclear strikes. Not only weren't they called madmen; Romney survived to become the nominee in 2012 and the presumed #NeverTrump candidate in 2016.
In 2008 John "Fit for the White House" McCain sang a song parody: "Bomb, bomb, bomb, bomb, bomb Iran." He has also said we could stay in Iraq for 100 years, has antagonized multiple world leaders, and has championed nearly every military intervention of the last 25 years (including Iraq, the biggest catastrophe of them all). He was almost the GOP nominee twice.
Mainstream GOP foreign policy has been rabid and unhinged for years. Trump's mistake on nukes is saying the "right" thing the "wrong" way. After Trump's defeat, the 2020 Republican hopefuls will continue the same fanged rhetoric without attracting the slightest notice.