What it says on the tin. How can we meet this challenge where no states secede from the Union even though we are only giving a dozen years to avert the phenomenon.
https://en.wikipedia.org/wiki/Treaty_of_Guadalupe_Hidalgo
No secession means no Civil War (at least not in any recognizable form). The 1848 baseline means that the geographic dimensions of the lower '48 are basically set.
Specifically, without secession and a Civil War, I'm interested in estimates of:
a) U.S. demography and population growth for the rest of the 19th century
b) The growth and size of the U.S. economy through through the rest of the 19th century
c) The US fiscal balance through the rest of the century
d) The role of U.S. shipping and other sectors in global trade
e) The fate of the Egyptian, India and Central Asian cotton industries without the significant interruption of U.S. supplied cotton
f) U.S. foreign policy and its international reputation