Sen. Elizabeth Warren, D-Mass., speaks to a conference as House and Senate negotiators try to resolve competing versions of a rewrite to the No Child Left Behind education law, on Capitol Hill in Washington, Wednesday, Nov. 18, 2015. (AP Photo/Jacquelyn Martin)

In a speech before the Senate Thursday, on the sixth anniversary of the Supreme Court's Citizens United decision, Elizabeth Warren made clear -- for those with ears to hear -- that she will not endorse Hillary Clinton.

If you have observed how closely Warren's and Bernie Sanders's messages line up, it is hard to imagine that she would endorse Clinton over him, anyway. Even so, the question has remained. But now, were there any question about whether or not Clinton is truly a Progressive, Elizabeth Warren -- with her extraordinary, precise eye for the heart of an issue, and her unsurpassed clarity of expression -- has answered it.

The first ten minutes of Warren's speech address corruption in campaign finance, and the impact of Citizens United. She lists seven steps we could take right now, including six actions -- bills before Congress, executive action, and powers already within the purview of the FEC and the SEC; and the seventh, a Constitutional Amendment to restore federal and state authority to regulate campaign contributions.

Warren is eloquent, moving, and on topic as always. Right at the end, however, she changes gears. I almost missed it; what she had said up to that point was so compelling that my mind was ringing. It was only on the second listen that I caught them: three sentences that leapt from the specific (campaign finance reform) to the general (Progressivism itself):

A new presidential election is upon us. The first votes will be cast in Iowa in just eleven days. Anyone who shrugs and claims that change is just too hard has crawled into bed with the billionaires who want to run this country like some private club.

It would be hard to overstate the controlled vehemence and contempt with which Warren delivers her last statement, just as there is no question who that "anyone" refers to. Clinton, unable to win over Progressives (both MoveOn.org and Democracy for America have endorsed Sanders), has attempted to reel in moderates by casting herself as the more pragmatic choice, and has painted Sanders with the broad brush of unrealistic idealism. Warren's message, so aligned with Sanders's as it has always been, is covered by those same strokes. Never one to shrink from a challenge, Warren comes out swinging.

Her riposte could not be more direct, as she reminds us of what true Progressives sound like and stand up for: broadly popular ideas, which are still somehow considered politically impractical. At the same time, she reminds us that 'politically impractical' is just code for 'wealthy donors don't like it.' The money Clinton has received from Wall Street, so much a topic of last Sunday's debate, illustrates Warren's indictment all too well.

Although the occasion for her speech was the anniversary of Citizens United, in mentioning the election and the imminent voting in Iowa Warren leaves no doubt that her closing words are meant for that greater context, even as she identifies Clinton's appeals to pragmatism as a complete betrayal of the Progressivism she had once courted. That may well be the ball game for Clinton; having failed to win over Progressives, Warren's endorsement could have shored up Clinton's eroding support long enough to survive the Iowa Caucuses. Instead, Warren has delivered a scathing rebuke.

It will be, at best, a long, drawn-out primary fight. It is likely Sanders will only pick up speed, though. Given recent extraordinarily staunch support for Sanders from powerful voices within the African American community; given Sanders's recent poll numbers, which have so astonished politics as usual, and which will lead many to reconsider his purported unelectability; and given many voters are only just now starting to pay closer attention, at a time when Sanders is surging like never before - it might well be a Bernami.

Whatever happens, one thing's for sure: Warren won't be endorsing Clinton.