After staying neutral throughout the Democratic primary, Elizabeth Warren endorsed Hillary Clinton for president on Thursday. In an interview with Rachel Maddow on MSNBC, Warren said that she's "ready to get in this fight and work my heart out for Hillary Clinton," and, in doing so, prevent Donald Trump from getting "any place close to the White House." Warren had come under criticism by some progressives for taking so long to issue an endorsement — but there are good reasons she waited so long.
"I'm ready. I'm ready to get in this fight and work my heart out for Hillary Clinton to become the next president of the United States," Warren told Maddow, "and to make sure that Donald Trump never gets any place close to the White House."
Maddow immediately asked her why she waited so long. Warren gave kind of a half-answer, stating that "the primary was really important [and] an opportunity to show this is what it is to be a Democrat." The implication is that she felt the primary process itself was valuable to the party and didn't want to interfere. In all likelihood, though, Warren's calculation was a bit more complicated than that.
As soon as it became clear that Bernie Sanders and Hillary Clinton would be running against each other, Warren was in a tough spot. On the one hand, she's much more ideologically in sync with Sanders; collectively, the two of them represent the progressive economic wing of the Democratic Party. On the other hand, she had publicly urged Clinton to run for president a year before the 2016 primary even began, long before there was any indication Sanders was thinking about a presidential run. Complicating the matter was the fact that, despite Sanders' breathtaking level of support, Clinton was always the clear favorite to win the nomination.
This put Warren in a bind. For Warren to endorse Clinton over Sanders would not seem ideologically authentic, and would furthermore tarnish her reputation among liberal supporters back home, thus imperiling her reelection chances. Backing Sanders, however, would contradict Warren's earlier support for Clinton. Moreover, it would risk making an enemy out of Clinton, who was (and still is) likely to be the next president of the United Stats.
This dynamic remained the same more or less the dynamic throughout the primary. For Warren, the best option was to simply stay out of the primary and avoid making any friends or enemies.