Major cities with Republican enclaves vs. ones without

[I spent the winter writing songs about getting better]

What exactly is the reason that results in this?

For example we all know about Philadelphia's Republican areas thanks to many. And then there's NYC even if you exclude Staten Island which has many noticeable Republican areas (even in the Bronx believe it or not.) On the other hand Minneapolis has none, there are no parts where the Republicans can even be competitive. Chicago is largely the same way. San Francisco just goes without saying. Places like DC and Detroit are obvious, but what causes some cities to have pockets of Republicans that many just lack?

[I spent the winter writing songs about getting better]

Blue-Collar Whites
Police

We have both of those.

It will be to do with the structure of the city (in terms of economy, society and housing) and civic culture, basically. And sometimes boundaries, though not in the case of any of the places you mentioned.

I specifically left out places like Jacksonville and Phoenix with a bunch of sprawl within the city limits.

[I spent the winter writing songs about getting better]

Something I have noticed is that downtown Minneapolis is non-negligably more Republican than the city in general (it was about 75% for Obama.) I've figured that's due to rich people living there, plus things like someone making a ton who works at a right wing law firm might prefer to just get a condo downtown next to work than a McMansion out in the exurbs. But it's not to any significant level or even swingable. Interestingly the U of M dorms are some of the most strongly Republican precincts in the city thanks to College Republicans from the suburbs.

[I spent the winter writing songs about getting better]

Something I have noticed is that downtown Minneapolis is non-negligably more Republican than the city in general (it was about 75% for Obama.) I've figured that's due to rich people living there, plus things like someone making a ton who works at a right wing law firm might prefer to just get a condo downtown next to work than a McMansion out in the exurbs.

Not shocking. But there is no such thing as a "right-wing law firm". Except perhaps something like the Texas firms specializing in "natural resources" (i.e., oil) law.

I'm thinking of ones that mostly specialize in rather shady but well-funded corporate clients.

[I spent the winter writing songs about getting better]

It was an extreme oversimplification, but what I was thinking is that someone who lives downtown to avoid a commute is far more likely to work at a law firm or in banking, marketing or some type of venture capital firm as the people in the dense residential areas. Now obviously there are still tons of Democrats working at such places (not to mention the type of people who make hordes of money in tips at trendy restaurants or bars, and the owners of said businesses who I'm sure vote Democratic.), but they are far more likely to be Republican than the mostly dense but residential neighborhoods who are likely to have more municipal employees, teachers and university employees, service industry, blue collar or tech-oriented jobs. Racially similar areas in Uptown tend to be about 85% for Obama as opposed to the 75% downtown area.

[I spent the winter writing songs about getting better]

Orange County is completely suburban and not part of a major city at all. It has nothing to do with this conversation.

Staten Island technically does qualify but as I sort of implied in the OP, it's not really the type of place I'm referring to. I'm referring to actually urban places that are willing to vote Republican, not suburban ones that just happen to be in the city limits.

[I spent the winter writing songs about getting better]

What's the most densely populated, solidly Republican area?  Subjective as to what "densely" & "solidly" mean, naturally.

The answer is probably Staten Island actually. I doubt there was a denser county in the country to vote for McCain. Those Russian-Italian neighborhoods in Brooklyn are probably denser though.

[I spent the winter writing songs about getting better]

I don't know NYC neighborhoods that well. Just look here (of course this uses the incorrect color scheme):