The median voter theorem was developed by Harold Hotelling in 1929, although it wasn't until the 1970's that the Democrats took it seriously. It states that "a majority rule voting system will select the outcome most preferred by the median voter".
His thinking was to compare politics to businesses selling similar consumer products.

Simple, easy to understand, and absolutely wrong.

“When parties fail to show ideological differences, centrist abstention is higher,” he wrote in the article, “When the Kingmaker Stays Home,” which was published in the journal Party Politics.

Democrats believe in their hearts a common paradigm:

"Republicans on the right, Democrats on the left, independents/swing voters in the middle".

To which the proper reply is:
"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."
- Mark Twain

The first thing Dems get wrong is where independents are.

Libertarians and Post-Moderns are largely white, well-educated and affluent. They also share a relatively secular outlook on some social issues, including homosexuality and abortion. But Republican-oriented Libertarians are far more critical of government, less supportive of environmental regulations, and more supportive of business than are Post-Moderns, most of whom lean Democratic.
Disaffecteds, the other main group of independents, are financially stressed and cynical about politics. Most lean to the Republican Party, though they differ from the core Republican groups in their support for increased government aid to the poor.

Now we could debate all day what this or that label means. That's what they do on TOP.
I want to break this down to election results.
To do that I want to look at the three major Democratic groups:
Blue Dog Coalition (fiscal and social conservatives)
New Democrat Coalition (fiscal conservative, social liberal)
and Congressional Progressive Caucus (liberal to leftist)

This table doesn't include retirements or already elected members joining the coalitions.
Someone else could break that down.
What stands out is how consistent the Progressives are, while the Republican-lite Blue Dogs account for most of the losses by the Democrats since 2010.