I've heard the calls for Texas, NH, MT, AK, and ME.
I think each has its merits, but each is ultimately untenable whether for economic, societal, geographical, or psychological reasons.
I would like to propose Michigan.
If population density is an issue, we can limit ourselves to the upper peninsula, which is the most likely large part of any state to change political alignment - much more likely than a couple of counties in South Texas with expansive land borders.
Geographically - Its too cold in the winter and too hot in the summer, but is not exposed any natural disasters save an occasional tornado or blizzard. It has no exposure to sea-levels, if those were to become a problem, and it is relatively insulated from global warming effects, however they are being predicted to arise.
Aside from Alaska and HI, there is no land more easily severed from the outline of the US than Michigan.
Politically, traditionally very blue Union cities, very red Farming country.
Michigan has been the Canary for the US, and she's about dead. US Steel policy and NAFTA(IMO) were the top contributers to the fall of the auto industry. The blue collar base has been used and abused here, and are ready for something else (we just need to convince them that there is less risk in joining our cause than just voting for the new promise-maker).
There is going to be an up-tick in the farming market here this year - barring a wide loss of the Sugar Beet crop. The cities will continue to lose population and money, the suburbs will, for the first time since the 50s, stop growing.
The only strategic limitation is negotiating passage of the Erie Canal to maintain access to the sea.