American politics has been described as a two party system, but it is really a one and a half party system. One party creates a winning political coalition while the minority party has to deal with the remains. Sometimes the minority party wins major elections, but it is generally rare.
There are two ways the parties shift or reorganize.
The first is when one party collapses. Most recent examples were Republicans in 1932 and Democrats in 1968. The majority party messes up so much that it is blatantly apparent that they aren’t going to win elections any more and the transfer of power is rather peaceful.
The second is due to mainly demographic change as the majoriry party becomes the minority party. This is generally more risky for democracies as the shift is gradual and there isn’t a firm reason parties to switch. There is also more electoral chaos as votes are a lot closer.
One example was the Election of 1800, where Southern and Westward expansion in the country had led Jefferson and Burr to beat Adams, but then there was a stalemate on who would be picked. While a political solution was found in the end, militias were being drawn up and there was a growing threat of civil war.
The second example was the Civil War. While Lincoln won a minority of votes, it was apparent that future elections would likely be won by the party of thr North due to increasing immigration.
It seems like Trump is tapping into this demographic fear. More educated voters vote Democratic and the populace is becoming more educated. Outside of Florida, it appears that voters are choosing to move to Republican Sun belt states and voting Democrat. What was thought of as a party collapse in 2008 is turning out to be a demographic shift instead. So, the former dominant party is fighting back to delay or prevent the shift.