The big tent party from roughly FDR until Reagan averaged 45% of the electorate.
https://www.pewresearch.org/politics/feature/party-id-trend/
From Reagan until Obama the democratic party was still the largest party, averaging 35%. Today the democratic party has shrunk down to 28% of all America. The republican party has finally pulled even with them having averaged around 28% from Eisenhower to today.
https://news.gallup.com/poll/15370/party-affiliation.aspx
The litmus tests from both major parties has seen a rise of independents, the non-affiliated, the less to non-partisan group of voters from 30% of the electorate in 2006 up to 43% today. I think the litmus tests developed by the democratic party over the years is the main reason for their shrinkage. The same for the GOP, their litmus tests have prevented that party from increasing their percentage of the electorate.
The problem, even though independents, the non to less partisan group of voters now make up a plurality, because of our two-party system, they must choose between two sets of litmus tests every election even though they dislike both. These litmus tests are the reasons they left the democratic party along with being the reason they refused to join or become a republican.
About that bright future I mentioned. I expect the democrats to regain control of the house in the 2026 midterms. Mainly because most Americans don’t like the litmus tests the republicans are now forcing on us. That the democrats will regain the presidency in 2028, but promptly lose the house in the 2030 midterms as most Americans will have become very dissatisfied the litmus tests being forced on them by the democrats. Then look for a republican to occupy the white house after the 2032 election. Also look for independents, the non-affiliated, the less to non-partisan group to be the majority of 50% plus by 2032 as both major parties continue to shrink.