This is what prevented a President Humphrey, although he nearly overcame his party split like Truman did. From WW2 to 2008, we had more years of Republican control of the White House than Democratic, and the civil rights issue was a key factor in the breakdown of the New Deal Coalition. We may be observing another shift of the same magnitude, just with the opposite party taking the losses.
If a third-party takes 10% of the R vote nationally (vs. 2012 results), Ds win Georgia, Missouri, North Carolina and are competitive in Mississippi, Arizona, Alaska, Montana, Indiana and South Carolina. I don't see why any conservative would want such a result. The proposed 'hung election' seems unlikely.