Senator Cory Booker (D, NJ) made it explicit a couple days ago in a speech opposing unanimous consent passage of some bipartisan bills, including one led by his Progressive-Democrat colleague Catherine Cortez Masto (D, NV). Booker’s position, loudly and proudly stated on the Senate floor:

It’s time for Democrats to have a backbone[.]
…
This is a problem with Democrats in America right now. We’re willing to be complicit to Donald Trump to let this pass through, when we have all the leverage right now. When are we going to stand up as a body and defend our work, defend our jurisdiction, defend this coequal branch of government?

Then, when challenged on this by Cortez, instead of responding with facts and logic, Booker answered in typical Progressive-Democrat fashion:

Don’t question my integrity, don’t question my motives[.]

Those Progressive-Democrat Senators who disagreed with Booker and his behavior, Senators Masto and Amy Klobuchar (D, MN), are increasingly on Party’s fringe, far from the Party center. That’s where the Bookers of Party stand.

That stance: everything against Trump and Republicans, nothing for what’s explicitly good for their constituents or for our nation as a whole.

Party’s core and only policy is naked opposition to all things originating outside of Party.