There is much talk about the a modern rise of disunity in America. Let's think about this.
First, we set some definitions. There may be some disagreement in the exact wording, but I use these definitions so that we can talk about concepts later by just using the word without lengthy explanations about what is meant.
Nations are based on people who identify themselves as a group and honor common attributes. Most people think of a nation as people (often with a common ancestry) who live together because they desire to have many things in common. Most people assemble themselves together according to attributes that are most important to them individually. These common attributes can either be voluntary or enforced by the group.
If you talk about a people, like say the Irish, you can also see it as the Irish nation. You can often identify an Irish person by their common attributes, even away from their group.
Countries are defined lands, usually demarcated with established borders respected by most people. There really isn't much more to say then that. Borders get drawn and everything inside those borders are defined as a country.
As an example, there is a country called Ireland and it has set boundaries that others tend to respect. Many countries share a name with a nation that predominates there.
A nation-state is the combination of a nation and a country. There have been many nation-states in the world. Until recently, most of Europe was composed of nation-states. In many cases, a nation defines its own country. This is often called a nation-state. The people (nation) defends their country against invasion by others. A nation-state also enforces the attributes that define people within its borders, often providing the policing of its nation members (citizens).
As an example, Ireland is a nation-state that is populated (mostly) by the Irish people (nation).
The United States of America (USA) was formed by the union of several British colonies. Each of these colonies were very distinct, looking like their own Nation-States and often with common attributes enough to look like separate nations. At its formation, the USA came together under founding principles to which each colony subscribed. As each colony became states in this new union, they did not surrender their distinctiveness as separate peoples (nations) or their individual borders. A federal government was created originally to address the common needs that all the states within the new country borders finally decided that each would share, such as mutual defense.
There was never a consensus about ideas like liberty or independence or even the oppression of the English king. People in the colonies had individual reasons (or compulsions) for being on the American continent. This was not a commons-based unified nation in any sense at all and so could never be a nation-state by definition. It was disparate groups that ended up on the Anglo-American coast-lands with the barest of any proximity.
To highlight the disunity between the colonies/states, the original Bill of Rights was a hotly debated catalog of powers that the states wouldn't hand over to a federal government and restrictions on its authority. The Constitution itself was the second formation of government in the USA, after the loose confederation of powerful states nearly fell apart . The people within the USA never turned into the "melting pot" that social engineers wanted to see, but people understood that only be learning to live side-by-side with others could this new country be strong enough to defend itself from invaders.
The USA was and largely is still just borders (with fading need to expand them), a military, and not much in common beyond that.
Everyone has an opinion on immigration, though often unspoken in politically charged discussions.
It has always been true that if a person could get into America, they were simply here and could basically stay as long as they liked. If some authority goes on a illegals witch-hunt, just hide out for a few years and it will blow over. "Citizenship" can be skirted as over 6 million working people have done. Major corporate leaders have managed to rise in America while keeping their illegal status quiet.
Only with the advent of social security did there become a schism between those who (had to) pay into the program (legals) and those who (were prevented) not pay in (non-legals). "Immigration" is mostly to do with taxation and employers wanting to avoid paying their legislated portion of payroll taxes. Entrenched power-brokers want no restrictions to foreign worker entry and the populace get their bleeding hearts pressed into that service.
Everyone can basically still come to America and no effectively powerful forces who could prevent this is interested in doing so (except Trump).
If you want to know, Trump is hated because he might tear down the "migrant" game and he won't work with the powerful to keep feeding off of it. Lies and loyalties will be twisted however is necessary to make sure Trump is made effectively toothless.
America welcomes everyone - it always has and it always will, walls be damned.
I am sure there are a number of prognosticators and statisticians who can produce data to say that border-jumpers will be more likely to vote for Democrats, especially if they are given a lot of free services. I think it is a very risky action, but I don't think the party leadership sees any cost to them if they happen to be wrong.
Then, sanctuary cities start getting thousands of these and begging for help to provide more free benefits to them which detractors portrayed as coming at the expense of citizens. Leftists damning all costs and Rightists ineffectively trying to stir up resentment.
All that has happened in the years following the Revolution is that disparate people have moved about, new disparate people have migrated in, and everyone mixed freely until states have no unifying ideas within their borders at all and all seem very generic.
In fact, we had a "civil" war that was fought to make sure the last few principles (attitudes about slavery) that might unify people within states were destroyed. Up until the destruction of the South, people considered themselves citizens of their individual states - "I'm a Georgian" or "I'm a Virginian". Come 1870, people associated less and less deeply with their (destroyed) states. It was the birth of "Americans" and living next door to a neighbor who has very little if anything in common with you besides a label
The only difference between states these days is how quickly they are offering benefits and attracting business, jobs, and people to them - the eternal race to the cultural bottom. That is the commonality that seems to unite the USA now.
Some people in America want an encompassing social structure that takes care of them. Other people want to be far from others and do their own thing without interference. Just as much as one faction wants to dominate governments to force everyone into their vision of a utopia, another faction wants to tear down governments until it leaves their compatriots unmolested. Everyone knows that these people cannot both get their way without some division, such as a new civil war or succession.
There is another idea that might work. It is a return to the colonies, sort of. It will need many people to move close to people among whom you fit. The federal government needs to be gutted and forced back into the cage of the original Bill of Rights - borders, a military, and nothing else.
It would be great. No need to go to war or break away. If you want socialism, you move to California. If you want near-anarchy, you can move to Wyoming. If you want to be among Catholics, move to New Mexico. If you want to be among Mormons, move to Utah. Each state would be its own particular flavor and mix, which no one tries to make like someplace else. Respect the predominant culture of the state in which you live (or move elsewhere). This presupposes that each state will support a culture that is their own and many new states will be formed from the "silly square states" (most states west of the Missouri river) to meet the unifying needs of individual groups, just like the original British colonies. Of course, that will require people to put culture and place before chasing cash, which is hard to do!
Here is a video of a lot of effort to help such an idea along, based on current realities: What If Every State In America Had Natural Borders?
There may need to be a constitutional amendment to create a method of reformulating more authentic states based on the idea of nations and provide critical masses for a new state's formation and survival. There will be major opposition but we will see who are about exercising power over others and exposing their evil intentions. I think it might get done and we can revivify the idea of the union of disparate nation-states that can come together enough to unite around helping your differing neighbor and defending the whole.