(CNN) — The stakes were high when Bernie Sanders and Elizabeth Warren met at Warren’s apartment in Washington, DC, one evening in December 2018. The longtime friends knew that they could soon be running against each other for president. The two agreed that if they ultimately faced each other as presidential candidates, they should remain civil and avoid attacking one another, so as not to hurt the progressive movement. They also discussed how to best take on President Donald Trump, and Warren laid out two main reasons she believed she would be a strong candidate: She could make a robust argument about the economy and earn broad support from female voters. Sanders responded that he did not believe a woman could win. The description of that meeting is based on the accounts of four people: two people Warren spoke with directly soon after the encounter, and two people familiar with the meeting.