Sen. Elizabeth Warren (D-MA) has been speaking to Hillary Clinton behind the scenes throughout her presidential bid, according to sources who spoke with NBC News.

It was widely reported Clinton met with several potential 2020 candidates at the beginning of the year, including former Colorado Gov. John Hickenlooper (D), Sen. Cory Booker (D-NJ), and Sen. Elizabeth Warren (D-MA). However, the failed presidential candidate has been speaking to Warren in recent months, according to sources who spoke to NBC News.

Sources are not sure how many conversations Warren has had with Clinton, and neither has made obvious attempts to make their conversations nor the nature of them known to the general public.

NBC News reports:

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.

“That has clearly not gone unnoticed, and I think she really appreciates that,” the person close to Clinton said.

It is established both women want President Trump out of the White House, though it is possible that Warren has failed to make her points of contact explicitly known due to the optics, as Clinton – another female who attempted to break the glass ceiling – lost handily to Trump in 2016.

“To the extent that Democratic primary voters fear a repeat scenario in 2020 — and to the extent that she’s competing with Sanders for the votes of progressives — there may be good reason for Warren to keep her distance from Clinton publicly,” NBC News reports. It notes Clinton could be a valuable asset in the event Warren “finds herself battling for delegates and superdelegates” at next year’s Democratic National Convention in Milwaukee, Wisconsin– the same state Clinton lost in 2016.

“Hillary Clinton would absolutely have influence over a number of delegates to this convention,” Deb Kozikowski, the vice chair of the Massachusetts Democratic Party, told NBC News.

“It would be counterproductive for the first woman nominee of the party to not be supportive of a woman who may go over that threshold,” Kozikowski added.

According to a Democrat strategist who spoke to NBC News on the condition of anonymity, Clinton admires Warren’s campaign.

“She has applauded her about being serious and disciplined and loves that she is sticking to her guns,” the strategist said.

Warren’s team would not provide NBC News with details on Clinton and Warren’s relationship or their conversations, but the outlet admits that the duo’s contact “appears to have been more than a courtesy call or a trip to the principal’s office.”