The two women have kept a line of communication open since the Massachusetts senator decided to run for president — though only a conversation around the time of Warren’s launch has been previously reported — according to several people familiar with their discussions who spoke to NBC on the condition of anonymity because of the political sensitivity of private interactions.

It’s hard to know exactly how many times they’ve reached out to each other — or precisely what they’ve discussed — in part because neither camp wants to reveal much of anything about their interaction and in part because they have each other’s phone numbers, and there are many ways for two high-powered politicians to communicate that don’t involve their staffs.

One source was aware of just one additional call between Warren and Clinton since then. But a person who is close to Clinton said the contact has been substantial enough to merit attention, describing a conversation between the two as seemingly recent because it was “front of mind” for her.