(CNN) "Yes."

That was Massachusetts Sen. Elizabeth Warren's response to a simple question asked by CNN's Jake Tapper Thursday night: "Senator, do you agree with the notion that (the 2016 Democratic primary) was rigged?"

And with that single word, a largely behind-the-scenes feud -- over not just how the 2016 election was lost but also where the Democratic Party needs to go before 2020 -- broke out into the public eye in a major way.

First, some context.

Warren was responding to book excerpts published in Politico Thursday by Donna Brazile, the chair of the Democratic National Committee, detailing the final months of the 2016 campaign. In the essay, Brazile alleges that the DNC was wholly owned by Hillary Clinton and her campaign prior to the party's presidential nomination being decided -- the result of a fundraising agreement cut between the party committee and Clinton's team.

Writes Brazile:

"The funding arrangement with [Hillary For America] and the victory fund agreement was not illegal, but it sure looked unethical. If the fight had been fair, one campaign would not have control of the party before the voters had decided which one they wanted to lead. This was not a criminal act, but as I saw it, it compromised the party's integrity."

In essence, Brazile is saying that because Clinton helped raised massive amounts of money through state parties for the DNC, the candidate and her team were given operational control over the party committee before Clinton was the actual nominee.

The Brazile allegations confirm the long-held suspicions of Sanders supporters who were convinced during the campaign that the party establishment's thumb was always on the scale for Clinton. And that the party apparatus worked to keep Sanders from having a fair chance at winning the Democratic nomination because, well, they didn't like him.

The forces aligned with Clinton and those aligned with Sanders have been sniping at one another ever since the 2016 primary ended that June. While they have found a common enemy in Trump, the ill will has never really gone away.

The Clinton people -- up to and including the candidate herself -- believe Sanders' attacks on her during the primary hurt her chances against Trump in the general election.

In her account of the 2016 campaign "What Happened," Clinton wrote that Sanders "had to resort to innuendo and impugning my character" because they largely agreed on the issues. She also wrote that Sanders' attacks inflicted "lasting damage" on her campaign and opened the door for Trump's "Crooked Hillary" label.

The Sanders crowd, on the other hand, thinks Clinton won only because the system was rigged against him. The leak of hacked emails from top officials at the DNC on the eve of the Democratic convention in July 2016 seemed to confirm some of those suspicions. The staffers questioned Sanders' religiosity, attacked Sanders' campaign manager Jeff Weaver and leaned in favor of big-dollar donors rather than the small-dollar army Sanders had built.

DNC chair Debbie Wasserman Schultz was forced to leave her post as a result. But in the midst of the general election campaign, Sanders, Weaver and most of the other top staff on Sanders' campaign urged the party to move on.

"This happened, we knew it happened then, now is the time to go forward," Weaver said on CNN at the time. "Now is the time to elect Hillary Clinton and defeat Donald Trump."

To have Warren -- someone that liberals pleaded with to enter the 2016 race and who remains a folk hero in the progressive community -- confirm in no uncertain terms that she believes the whole Democratic primary fight was rigged is stunning. Sanders hasn't even said that!

This all matters hugely because Sanders and Warren are two of the favorites to be the Democratic nominee in 2020. Both are voices insisting that the party needs to confront Trump and Trumpism at every turn, back things like single-payer health insurance and generally embrace the word "liberal."

Brazile's book -- and Warren's "yes" -- amounts to spraying a can of lighter fluid onto a bonfire burning between liberals and the establishment. And it's only going to get hotter from here on out.