In competitive elections, the Rust Belt won't be carrying for the Republicans while their base is the Old Confederacy. People arguing that it is feasible aren't being realistic.
The 1960 Presidential election disagrees with you. The Democrats were able to carry several of the big Rustbelt States, as well as a lot of the Southern States.
1960, along with 1968, were competitive presidential elections in which the two major-party nominees were able to carry select states normally carried by the other party.
What you're mentioning is not common. Lyndon Johnson won in 1964 with his base of support from the north when, at the time, the Democrats had their base in the Old Confederacy. Richard Nixon's re-election in 1972 saw him carry all Old Confederacy states with margins exceeding his national number, back then opposite of Republicans' base.
If you think today's Republican Party is going to rebrand into the party they used to be prior to realignment, and that today's Democratic Party is going to rebrand into the party they used to be prior to realignment, you should depend on your stated example of "1960" to anticipate another uncommon cycle that serves as a preview for another realigning of the map (Old Confederacy-vs.-Rust Belt).
Well the, how about the 2004 Election? I assume we can agree that this serves as an example of a close election.
The Republicans obviously won all the states of the old Confederacy. In the Midwest they won states like Iowa, Ohio and Indiana. What is more interesting are the margins in some of the states that Bush lost. According to Wikipedia he lost Wisconsin by 0, 38 %, Pennsylvania by 2, 5 % and Michigan and Minnesota by around 3, 4 %. To me that is pretty close.
We have a lot of landslide victories for either party where the Midwest and the South voted for the same candidate. Then we have some close elections like 2004 and 1960 where the same thing (to a lesser extent) is happening. To me that suggests that there is little statistical data (at least after 1945) to support the idea that the Midwest and the South must necessarily belong to different coalitions.
