I wrote the following on February 22, 2016 and I expect the reasoning to hold up as Warren makes a very similar calculation in the wake of ending her presidential bid:
Speculation about when or if Senator Elizabeth Warren would endorse one of the candidates battling for her party’s nomination has been hot and heavy. Sanders’ backers, in particular, have been confidently speculating that Warren would eventually side with Bernie. The truth is that Warren will not endorse until the nomination is a fait accompli. She will not put her foot on the scale to help the now faltering protest candidacy of Bernie Sanders primarily because she believes in the Democratic Party. She believes that a strong united Democratic Party is the key to electoral victory and policy accomplishment.
Hillary Clinton’s glide path to the nomination appears to be back on track following her win in the Nevada caucuses. If South Carolina goes as planned, Warren will face mounting pressure to endorse Sanders in time for Super Tuesday. I fully expect her to resist this pressure.
Senator Warren’s role in this year’s Democratic presidential nomination process will be to unite the party around the nominee. While stepping in and endorsing Sanders before March 1st would certainly change the contours of the race, giving the Vermont Senator a serious boost, it would also create a deeper rift in the party with the potential to damage the nominee in the fall. For Warren, it would be a foolish gambit that could easily complicate her efforts to be a force for progressive change in the U.S. Senate going forward.
There’s no denying the ideological fit between Warren and Sanders, but there is also a significant difference between the two. Warren understands better than Sanders the limited utility of protest politics. She also understands that if Sanders truly is capable of winning in the fall, he would also be capable of winning the nomination without her foot on the scale.
The perception, repeated by Eric Fehrnstrom in the Globe recently, that Warren owes her own 2012 election to ultra-progressive voters and her “anti-Wall Street crusade” is flatly wrong. Warren was a good candidate who definitely put her progressive message front and center, but she beat Scott Brown because she is a Democrat, not because she is a progressive. Warren understands that the realization of her agenda requires a healthy, powerful Democratic Party. She may sympathize with Bernie’s ideology and values, but she does not sympathize with Bernie’s contempt for the Democratic Party establishment.
After Hillary Clinton has locked up the nomination, I expect Warren to begin counseling unity between Bernie’s raiders and Hillary’s partisans, and I expect a rousing, unifying convention speech from the Bay State’s senior senator reminiscent of the liberal icon whose seat she now occupies.