It's no secret that the Democratic Party would rather lose to a Republican than win with a progressive. The Democrats prefer to at least pretend that they embrace progressive values, and thus get those progressive votes...until now.

A few weeks ago the DCCC attacked a progressive candidate in Texas that was running against a union-busting lawyer and a Goldman Sachs banker. Democratic voters in Texas reacted by voting for the progressive candidate.
However, with the slim election victory of Republican-lite candidate Conor Lamb, the Democratic establishment totally forgot about the lesson from Texas and are preparing to take down progressives in California.

Facing the risk that the party could get shut out of the general election race for one or more competitive Republican-held seats, liberal groups formed to attack Republicans now say they are at least considering spending money to support particular Democratic candidates in the primaries. National Democratic officials say all options are on the table in the lead-up to June – including launching negative attacks on members of their own party, a tactic that stirred controversy in the Texas primary.
...
Katie Merrill, advisor to the Democratic Super PAC Fight Back California, still intends to target Republican incumbents in key California congressional races – its original purpose. “But if it looks like we’re in danger of a Democrat not advancing to a general election as we get closer to June, we might have to – and other groups might have to –focus on supporting a particular Democratic candidate.”
...Groups like EMILY’s List and Democracy for America also plan to support their endorsed candidates in several of the seven Republican-held districts.
An aide at the Democratic Congressional Campaign Committee (DCCC) confirmed that the party committee hasn’t ruled out supporting or even attacking a particular Democratic candidate in California, despite the backlash that produced in Texas last month.

Siding with a candidate before the primaries is controversial enough, but attacking another Democratic candidate is inexcusable.

When it comes to the Dem establishment, their preference is always toward the most Republican-like candidate, never once toward the progressive.
In case you didn't believe that, check out these recent articles.

Democrats have shot at flipping the House if they can stay in the center

The clear path to winning back the House is to recapture the votes of Trump defectors in red states and swing states by leaving behind polarizing positions and revitalizing the centrist Democrat.

Another round of Schumer's “For every blue-collar Democrat we lose in western Pennsylvania, we will pick up two moderate Republicans in the suburbs in Philadelphia, and you can repeat that in Ohio and Illinois and Wisconsin.”
Because that worked sooo well in 2016.

Democrats' 2018 midterm hopes strengthened by decline of liberal 'purity tests'

Rather, Democrats have begun to fall head over heels in love with the goal of defeating Donald Trump and his agenda. And to achieve that, it seems more and more Democrats may be willing to check progressive purity at the door in favor of progressive pragmatism.
...The sense was that these progressive perfectionists were ready to abandon the Democratic Party completely — or at least stay home during the midterm elections — if this didn’t happen
Flash forward to March 2018, and I’m now hearing some of those very same Bernie purists say that if the candidate has a “D” in front of their name, they will vote for them.

Considering that he refers to them as "Bernie purists" and "progressive perfectionists" I'm willing to bet $5 that this guy hasn't heard anything like that. He's made this up whole cloth to fit his agenda.

However, this article below is the one that tells you all you need to know.

Inside the Democrats' efforts to elect more Conor Lambs

The effort ramped up in the fall, when the Democratic Congressional Campaign Committee invited a group of moderate Democrats that put them in the majority in 2006 to talk through a winning strategy.
The committee gave the "Blue Dog mafia," as some call them, free rein to work with candidates on campaign operations in some of the toughest districts on the expanded Democratic target list.
The group, made up of former policy and campaign aides who now work at DC lobbying shops, consulting firms and industry groups, have set up their own satellite political operation to help vet candidates, determine who is viable to beat the Republican nominee in November, and provide support to build volunteer networks, set up digital operations, and make fundraising connections.

It just proves that the DCCC has learned nothing from the defeats of 2010, 2014, and 2016.
Absolutely nothing!