Every Eastern state from Connecticut southward that was winnable by a D post-1872 became a Republican overperformance zone except for Maryland, which moved from safe D to marginal. Republicans flipped every urban county from Boston to San Francisco, and carried New Jersey, a state that never voted for Lincokln, as well the previously Democratic NYC boroughs. This is what Walter Dean Burnham calls the "Systwem of 1896" and that Kevin Phillips discusses extensively in ERM and in his American Presidents Series bio on William McKinley (2003).
After 1896, Dems were confined to the Old Confederacy, sometimes KY-MO, the new state of Okla., and in 1916, Ariz and NM.
Dems won the WH in 1912 only because of the R party split, and held on in 1916 by a 3,773 vote margin from California.
You also said: "Each of these elections was characterized by one of the two parties ejecting the other from majority status and maintaining the White House for up to 24 consecutive years as the “sun” party"
The Republicans were the "sun" party from 1860-1928. Not only that, but the Democratic party was basically a party that tried to unify the south & the west to create an electoral majority.
You said "After 1896, Dems were confined to the Old Confederacy" This statement is as much true for the 1860-1892 period as it is for the 1896-1928 period.
http://www.presidency.ucsb.edu/elections.phpLook at the individual elections from 1896-1928
1896: the economy was in a shambles, the Democrats were all but guaranteed to lose that year (Like Van Buren in 1840 & Hoover in 1932)
1900: McKinley was a popular incumbent
1904: Roosevelt was an even more popular incumbent
1908: Taft was TRs hand picked successor
1912: Taft & TR split republican ticket
1916: "He kept us out of war"
1920: A reaction to Wilson's popularity
1924: Coolidge popular incumbent during "Peace & Prosperity"
1928: Hoover seen as Coolidge's successor & is one of the more popular Americans in his own rights (Both parties wanted to run him)
As you can see the Democrats had an uphill battle in all the elections they lost. In the elections from 1860- 1892, only Cleveland & Tilden were able to make serious inroads into the North.