Different Salutations Across Party Lines

If you want to see what names candidates use to refer to each other, check out this visualization on the New York Times website.

It seems that whenever republicans refer to a democrat, and whenever democrats refer to a republican, they use the informal Firstname Lastname format. When referring to a member of the same party, on the other hand, candidates use titles like Governor and Senator. Is this a new pattern or has this always been the case? Is it strategic or accidental?