Some kind of Sun Belt stack. IMO, a corridor of high-growth Western states battered by climate change, shaped profoundly by new Californian and Latin American residents, and the epicenter of a new kind of Democrat who is both hawkish on a level unseen since the neocons and somewhat progressive domestically in the name of Cold War II competition. This wall could also include Eastern strongholds like Virginia and Georgia, but I assume "Blue Wall" implies geographic continuity.
Like the original Blue Wall, margins vary, but Democrats can generally count on these states and always seem to win coin tosses (like the Blue Wall in 2004).
