There are many historical studies about the left/right composition of the two major US political parties since, say, 1900.
In short, until quite recently (about the time of Newt Gingrich's rise in 1995), there was a substantial overlap in the Venn diagrams of the two parties.
There were (comparatively) liberal Republicans, and (comparatively) conservative Democrats included in each party, with the obvious bias between the two centroids.
Historically, some of the conservative Democrats were Southern "dixiecrats", e.g. Strom Thurmond, who favored racial segregation policies, and some of the liberal Republicans were Northerners, e.g. Nelson Rockefeller, left over from Lincoln's time, when the Republicans were the civil-rights party.
Around 1960, a standard political joke (see "Beyond the Fringe") explained the US political scene to a Brit:
"There is the Republican Party, which is like our Conservative Party, and the Democratic Party, which is like our Conservative Party."
The first change was the civil rights legislation of 1964, which drove the segregationists away from the Democratic Party and led directly to unprecedented Republican majorities in the southeast quadrant of the US.
The most recent change, obviously, is the rise of Mr Trump to make much of the Republican Party his personal fief.
The next presidential election in 2024 may not change the policy stands of the two parties very much, but the personnel may change.