The progressive insurgency in the Democratic Party that began in 2016, and peaked in 2019, has been in decline for several years now. The party establishment has sensed it's weakness and now is looking to give the coup de grâce to the upstarts.

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.