Which states are Upper Midwest?

[cinyc]

The Midwest starts in Ohio and goes westward to both Dakotas. Why? Well, the original west has always meant the Northwest Territory, and when areas to the west were added to the U.S., it became the Midwest.

Why do I include the Dakotas but not Pennsylvania? The bulk of the population in the Dakotas is east of the Missouri River, and East River has more in common with Minnesota than Montana. Why isn't Pennsylvania or New York included in the Midwest? Because the bulk of people in those states don't live in the rust belt cities like Buffalo and Pittsburgh, and those states were not part of the original Northwest Territory. States and state lines are not arbitrary things that ought to be cast aside when determining which states are Midwest.

[cinyc]

I grew up in Denver which is clearly a Midwestern city with a large share of its population from the classic Midwest states of Kansas, Nebraska, Iowa, Missouri, and some Illinois. It has nothing in common with Cleveland. If any areas east of the Mississippi are Midwestern, it would be areas along and south of the National Road.

Well, the National Road went through the current capitals of Ohio and Indiana, so...

[cinyc]

Is there actually an Upper Midwest, or is a transmorgrification of the Upper Mississippi?

There are companies and media outlets that describe themselves as serving the Upper Midwest, and though I've heard that label most often in the region of the Upper Mississippi, I've never heard the description Upper Mississippi applied that way. I've only heard Upper Mississippi applied in the geological sense or to describe pre-Columbian cultures.

Have you ever heard the term Lower Midwest? Would someone from Kansas or
Indiana say they were from the Lower Midwest?

The North American Baptist Conference has a Upper Mississippi region (MN, IA, WI, IL).

I'm not saying that the area of the Upper Mississippi became the Upper Midwest, but the concept of "upper-ness" may have come from the river divisions, and of course there is clear distinction between Upper Mississippi and Lower Mississippi, except perhaps for the bit between St. Louis and Cairo.

538 ran a survey that agreed with my definition. Most Midwesterners agreed that the Midwest included their State and its neighbors.

Only about 10% of respondents in that 538 poll agreed that Colorado was Midwestern, as you have argued.  Yet a majority of respondents put Michigan and Ohio in the Midwest, the latter of which, at least, you've argued shouldn't be considered Midwestern. The greatest percentage of respondents agreed Illinois should be included, which to me is a no-brainer, as Chicago is the largest Midwestern city.