@Robert Gentel,
That's an interesting question. Assuming the candidates would be Sanders, Clinton, Trump and Rubio, Clinton would probably be the most centrist candidate but she has so much baggage that people might not look at her voting record or positions. IMO, the left is more receptive to outside candidates (Nadar got more than 5% of the vote in 11 states in 2000), but Trump has a lot of appeal to the nationalist wing of the Republican party.
In the end, I think it would help Rubio because I think Trump will continue to implode. As he plays to the radical right, I see those in the mainstream right I know are getting really upset. Around here, he has lost a lot of support. Rubio can be a kinder, gentler Trump and get his votes, but I think the Sanders crowd will stick with him.