Sen. Mitt Romney warned a "cornered and delusional" Putin could use nuclear weapons.
In a New York Times op-ed, Romney suggested strategies to face Putin in such a scenario.
Romney said continued support of Ukraine was necessary to avoid the subjugation of other countries.
Republican Sen. Mitt Romney suggested in a New York Times essay that "NATO could engage" in Ukraine, "potentially obliterating Russia's struggling military" as an option against Russian President Vladimir Putin were he to deploy nuclear weapons.
The Utah senator also suggested that the West confront China and other Russian-allied nations with an ultimatum. "You are either with us, or you are with Russia - you cannot be with both," Romney wrote.
"Russia's use of a nuclear weapon would unarguably be a redefining, reorienting geopolitical event," Romney continued. "Any nation that chose to retain ties with Russia after such an outrage would itself also become a global pariah."
Romney warned that a "cornered and delusional" Putin could use nuclear weapons in the war against Ukraine, citing warnings from former Secretary of State Henry Kissinger, Russian ambassador to the US Anatoly Antonov, and CIA Director William Burns.
Burns warned in April that Putin's "desperation" could result in the use of "tactical nuclear weapons or low-yield nuclear weapons."
Romney also encouraged the US to continue to support Ukraine's war efforts against Russia. If the US stopped sending weapons and pressured Ukrainian President Volodymyr Zelenskyy to concede to Russia, Romney wrote, Putin would continue "invade and subjugate" other nations.
President Joe Biden signed a $40 billion aid package Saturday to assist Ukraine; $20 billion will go towards military assistance.
Romney has been vocal about his disdain of Putin since the start of the war, referring to the Russian president in February as a "small, evil, feral-eyed man" trying to shape Russia into an "empire." He has also called pro-Putin GOP members "treasonous."
This week, Russian political scientist Alexei Fenenko told "60 Minutes" that Putin's invasion of Ukraine was a "rehearsal" for a larger conflict and that they were using the war to "test and go up against" NATO weapons.