I think from FDR until Reagan this country for the most part was a one party country. Democrats averaged 45% of the electorate to 28% for the Republicans. From 1933-1994 the Democrats controlled the House for 58 out of 62 years which included 40 straight years. They controlled the senate for 52 out of those 62 years, if not for Reagan 1981-1986 when the GOP controlled the senate, it would have been, like the house 58 of 62.
The Republicans were able to elect two presidents from 1933-1980, Eisenhower, everyone liked IKE to include Democrats. Nixon won in 1968 and Ford finished up his second term. Three presidents, but only two elected. During Reagan the Democratic Party share of the electorate dropped from an average of 45% down to 35% and pretty much remained there until Obama in which their percentage dropped down to 30%. The republicans rose to 32% during Reagan, then dropped down to 30% until Obama and has decreased to 26% today. Independents have climbed from 15% under FDR to 30% under Reagan until G.W. Bush then began to rise again in 2006 up to 43% today.
Those are the numbers. I think as the two parties, one drifting further and further right, the other further left especially since 2006 this has caused the rise in independents as they deserted their former parties. One or the other party can, has the opportunity to dominate in politics over the next 20-30 years if one or the other would move back to the center picking up those independents they have lost. To take charge of the country like the democrats did in the pre-Reagan era. I doubt that will happen though.
Independents now seem to vote against the party they are most angry with. The Republicans in 2018, the Democrats in 2010, The Republicans in 2006 and the democrats in 1994.