From Jeff Greenfield at Politico Magazine:
“I think a third candidate would be very likely on many state ballots,” [Former Romney strategist Stu Stevens] says. “ First of all, I think most GOP voters would want an alternative to vote for out of conscience. But Trump would also be devastating to the party and other GOP candidates. A solid conservative third candidate would give options to senators like Ayotte, Johnson and Kirk to run with someone else and still be opposed to Hillary. In fact, I think it’s plausible such a candidate could beat Trump in many states.”
Any candidate attempting a third party bid would confront serious obstacles, such as getting on state ballots late in the election calendar. As for down-ballot campaigns, most state laws prohibit candidates from running on multiple lines; so a Senate or Congressional candidate who wanted to avoid association with Trump would have to abandon the GOP line to rerun with an independent Presidential contender. The Stevenson [IL Gov. 1986] example shows that leaving a major party line is fraught with peril—although the write-in triumph of Alaska Sen. Lisa Murkowski in 2010 suggests that it can sometimes succeed.
The very fact that serious political thinkers are contemplating such a possibility demonstrates that when Republicans look at the perils posed by a third-party bid from Donald Trump may be looking in the wrong direction. It’s not Trump the Defector that could trigger the biggest threat to the party, but Trump the Nominee.