Warren sat on the sidelines for most of the 2016 campaign, refusing to back either Sanders or eventual nominee Hillary Clinton despite considerable pressure from both sides. She was the only Democratic female senator to skip a big endorsement event for Clinton, making headlines for her absence.

But once it was clear that Clinton would emerge as the nominee, Warren endorsed her and pushed to become her running mate. When she was passed over in favor of Sen. Tim Kaine (D-Va.), Warren kept pushing from the inside, for example preparing lists of liberals that she hoped the Clinton team would use to staff the new administration.

Some of Sanders’s supporters started the 2020 campaign still angry that Warren had not enthusiastically backed Sanders, her ideological ally in the party’s liberal wing. Warren moved further to the left after the 2016 campaign — co-sponsoring Sanders’s Medicare-for-all bill and backing his choice to lead the Democratic National Committee — in part to make amends to him and his backers, according to a person familiar with her thinking who spoke on the condition of anonymity because the person was not authorized to discuss internal deliberations.

Warren has been both a team player and an outside critic during her career.