I thought I'd do a comparison of to the population today's states, withe the population of those given areas in the Medieval America world. Now, the whole continent has plummeted by roughly four fifths, or 80% or so, however this is not an across-the board number. Some places have seen their populations plunge even deeper, while others have been relatively more relaxed in their decreases. (By comparison, Canada has decreased by 88 percent.) Figures for states largely inhabited by nomads are a little more difficult to pin down as, by definition, those populations will always be in flux. The Dakotas, Montana, Kansas and Nebraska are almost completely occupied by Herdsmen, so it would be hard to be fully accurate. (And then you have Nevada and Colorado, which flirt with being uninhabited) I've put asterisks next to states who have a mixed population of settled farmers and herdsmen. So what causes some of the once mighty states like Texas and California to fall so hard?
Probably the easiest to
pick up on, there are places in America there are just not as
hospitable without the heat or water, meaning populations of former
urban hubs like the Twin Cities or Phoenix were scattered to the winds.
But even places like Florida and Texas can be unbearable--much of the
southern population boom of the latter half of the 20th century was due
to the invention of air conditioning. Obviously, there's not a lot of mobility, so much of the deep south isn't going to completely dissipate, but there aren't going to be as many major metropolises--your Houston, your Phoenix, your Miamis, etc. Likewise, the states like New England or Minnesota have trouble feeding peoples. The Northeast's access to the sea and history of infrastructure keeps it from plummeting too much, but it's not mecca it once was.
Obviously, a less industrial society means a smaller population, but it also means smaller cities, and in turn, cities being a less significant portion of the population. Farmers make up to 90% of the population in medieval times. Cities in the modern period are able to defy natural limitations--make buildings taller, ferry garbage away, import resources. With this, we've seen a steep decline in much of the smaller (and first to industrialize) states of the the northeast, versus the larger states, rural of the deep south. New York City today has a population of 8 million--more than any former state, and most medieval kingdoms! If you remove New York City's population from that of the state, the decline goes from 84% to 72%. The former borders of New York still contain a healthy 3 million (the most outside the Ohio Valley) but there are simple geographic limitations. With a medieval level of technology, there's a much lower ceiling on how many people can live in a given space. With less emphasis on metropolises, states are now confined to the limits of 100 or so denizens per square mile. Compare New Jersey, which saw a heavy 90% decline, with Mississippi, which has the lowest. New Jersey, in the modern day, is the U.S.'s most densely populated state, absorbing much of New York and Philadelphia's metropolitan areas. Even in Medieval America the area that was once New Jersey is very densely populated, but it is very small--smaller than some New England states. Mississippi saw the most level decline, because it's very large, and already pretty rural--none of its cities are among the 100 largest in the country.
As you can see, I haven't even bothered to include much of the prairie states. One reason is that as the grasslands are now inhabited by nomads, even gauging the exact population is a futile effort. White estimates there are some three million herdsmen on the grassland. (With another million or so in the desert) With seven states that have completely turned to pastoralism, (and Wyoming all, but), we have 375,000 per state on average--probably less considering that doesn't include Texas and Canada. Not a huge loss for the Dakotas, Montana, or Wyoming (Which, during a busy enough season, may see occupation that rivals it present day population), but a heavy hit for the two farm states of Nebraska and Kansas, and especially Colorado, who set itself as the epicenter of the Rocky Mountains. Obviously, the biggest cause of deprecation out here is the oft-mentioned hordes of Cowboys, who won't let farms or cities dot their territories. However, it should also be noted that with less coasts or navigable rivers, transportation is much slower and potential immigration and trade networks aren't able to form the confluence that makes up cities. Likewise, with no oil industry or heavy mining reduces the importance such places had in the Industrial age. Ultimately, civilization peters out until we get to Salt Lake City and the rivers of the Southwest.
Rhode Island 92%
New Jersey: 90%
*New Mexico: 85%
New York: 84%
New Hampshire: 75%
North Carolina: 74%
South Carolina: 67%
West Virginia: 65%