(CNN) Less than two months ago, it looked as though Elizabeth Warren might just run away with the 2020 Democratic presidential nomination.

The Massachusetts senator had surged in Iowa and New Hampshire -- the two states that will lead off the nomination fight earlier next year -- and had pulled into a dead heat with former Vice President Joe Biden nationally.

Then that Warren wave hit a wall. Starting right around mid-October, Warren's numbers not only stopped moving upward but also began trending down. A Quinnipiac University national poll released just days before Thanksgiving showed Warren at 14% in the national ballot question -- down from 28% in the same poll in October. In a Des Moines Register/CNN survey released in November, Warren dipped to 16% -- down six points from a September poll when she led the pack.

Warren's slide is impossible to debate. What's more up for argument is the "why" behind it.

You can argue -- as Warren allies do -- that she was riding an artificial high over the summer and into the early fall. That she was never going to put the nomination away this early -- especially with such a giant field and the presence of an establishment favorite like Biden. And that while she isn't at the top of the pack right now, she's still in the top four and well-positioned to make a strong run at the nomination. (That last part, no matter how you explain Warren's recent struggles, remains, without question, true.)