A month ago Matt Grossmann and David A. Hopkins explain in Polyarchy Why Democrats have no “Freedom Caucus”. It’s a weak title but this is the least partisan and best explanation of the differences of the parties I’ve seen. Here’s the intro:
In our new book, Asymmetric Politics: Ideological Republicans and Group Interest Democrats, we explain why the congressional parties have come to behave so differently. Because the Republican Party is defined by its adherence to the symbolic cause of conservatism, Republican officeholders pledge fidelity to a set of abstract values.
In eras when conservatives largely agree about how their ideological commitments are best advanced in practice, the party appears relatively unified and harmonious. When significant internal disputes arise — a regular occurrence during the Obama presidency — they tend to take the form of accusations from one faction of Republicans that their fellow partisans have betrayed conservative principles.
The Democratic Party, in contrast, has consistently maintained the character of a coalition of social groups more preoccupied with pragmatically seeking concrete benefits from government than with advancing a larger ideological cause. Disagreements among Democrats tend to divide the interests of one group or set of groups from another.
In previous decades, when the coalition included white Southerners and conservative Catholics as well as racial minorities and left-leaning intellectuals, forging compromise was a particularly difficult task for the Democratic leadership. Today, the constituent elements of the coalition are more mutually compatible in their policy preferences, although party leaders must still work to satisfy the policy priorities of each group without the ability to appeal to a common ideological commitment.
They point out that restoring earmarks won’t help because the ideologues will still fight.