The answers so far posit a regional separation.
Look at other examples:
England with the war of the roses. (Rival claimants for the crown)
England during the Cromwell Period. (Protestant vs Catholic)
Irish troubles was/is nominally a conflict about Protestants vs Catholics, but under the hood is more a case of class warfare: By in large the Catholics were not property owners.
French Revolution was really class based civil war.
France 16th century: Problem between Huguenots (French Calvinists) and the established Catholics. The Huguenots were, I think about 10% of the population at it's peak.
States are tied together firmly with trade right now. To get a civil war, you need to go irrational: People have to be willing to take a serious economic hit out of rage our outrage.
Possible plausible scenarios:
Scenario 1. Regional based economic secession.
Trumpist style federal government cuts programs that benefit 'blue' states. California passes a law requiring that all taxes be paid to the State of California who then forwards payment to the federal government. Federal returns are no longer required. Enough Californians are pissed off with the federal government that the vast bulk of individuals and corporations co-operate.
Federal returns become an unenforceable law. As programs to California are cut, California reduces transfers to Washington.
Washington, Oregon, many other blue states follow suit. Neighbouring states amalgamate many services. Non-neighbouring states form a federation.
This can develop in various ways:
A: Becomes the new norm. While nominally part of the U.S. California and like states have in effect severed their political ties. The degree of trade with the remains of the U.S. can vary depending on your scenario.
B: Same thing happens internally in some states. California, Oregon and Washington rural areas east of the Cascades/Sierras attempt to separate from the state. Most will try to negotiate becoming part of the adjacent state (Nevada, Idaho) This results in a patchwork at the country level. Depending on how the adjacent states get along, this may become an uneasy truce, or a bunch of essentially self sufficient fiefdoms.
C: Is resisted by the remaining states. This is complicated by the number of defence establishments that are in blue states. The western seaboard is essentially cut off at the passes over the coastal mountain ranges. Many red state units called up are very short staffed, as lots of individual soldiers aren't happy with shooting at other Americans.
Overall military units using high tech gear quickly run out of material. Too many things are produced in multiple locations, and assembled on a Just In Time basis. Many red state units are essentially decapitated, with largely liberal officers refusing orders, or establishing 'white mutiny' or other forms of non-compliance, passive resistance.
D: The Trumpist Government uses nuclear weapons against recalcitrant cities. Given the distribuition of nuclear weapons, this doesn't end well.
Scenario 2: Religious/Racist based civil war.
In this scenario the irrational fundamentalists / disaffected trump style supporters decide they are going get rid of the polluting dark skins/catholics/jews/. The scary bit about this, they have a large fraction of the firearms. It starts with a single mid sized city in the rust belt passing a law that certain groups have to leave. They are given a short period to do so. A local militia shoots/hangs/burns alive outspoken members who protest. The state governor who hails from that city decides to see how this will play out, and does NOT call out the National Guard to restore order. Media coverage by social media and drones is very thorough. This instantly polarizes the people especially in the red states. Other cities follow suit. Small towns put up signs: "No Niggers, spics, chinks, papists or kikes allowed in city limits after sundown." Catholic churches, and baptist churches with largely black congregations are burned.
Like the Rwanda massacres, people are given the choice: Kill that (other) or die.
Telling the players apart is difficult, at least telling a white racist from a white tolerant person is difficult. There is no obvious class, language, or accent difference.
This will NOT be on a state level, but will be at a much finer granulation. Take a look and red vs blue counties in the last election. For examples look at Myanmar, Rwanda, and Cambodia.
second civil war site:goodreads.com. $\endgroup$