When Bernie Sanders debated whether to enter the 2016 presidential race, one of the first people he consulted was Sen. Elizabeth Warren (D-Mass.).

One of us has to run, Sanders (I-Vt.) told his Senate colleague at the time, according to a source familiar with the conversation.

Years later, with a new Democratic primary approaching, both senators are mulling whether to jump into the race.

And some Democrats say they don't think there's room for both progressives in what is expected to be a wide-open but crowded Democratic contest to take on President Trump.

"If a likely winning strategy is to reconstitute as much of the Obama and Clinton coalitions as possible, it's unlikely they will be able to do so sharing the same ideological space," said Democratic strategist Basil Smikle, name-dropping the last two Democratic presidential nominees.

Sanders, 76, and Warren, 69, have much in common.

Both are heroes to the left and represent the progressive side of Democratic politics that clearly has momentum within the party.

They have complimented one another and joined forces on common causes, most recently last week on legislation that would provide debt relief to storm-ravaged Puerto Rico.

"They align on almost every issue," one Sanders ally said. "That's not an accident."

Yet sources say tensions lie beneath the surface, including over Warren's decision to remain neutral in the 2016 Democratic primary between Sanders and Hillary Clinton, a more centrist politician.

"There's definitely a built-in rivalry there and there's definitely some unspoken tension," said one Sanders associate, who worked on the senator's 2016 presidential bid.

If the two both make a charge for their party's ring in 2020, it will be difficult to avoid raising tensions further.

"Bernie feels he did run and did enormously well and he's earned the right to run again," said the Sanders associate, who put it on Warren to determine whether she wanted to take on Sanders.

"She's got to figure out if there's room for the two of them. He's the one who was there, has an organization, still has his following and he's still getting huge crowds wherever he goes," the associate added.

Strategists say Sanders and Warren will be hard-pressed to share the same lane, leading to speculation that if both run for the White House, one will have to run to the center, however slightly.

"Someone, most likely Senator Warren, would have to move a little to the center to be successful in a primary and general," said Smikle, who worked for Clinton and served as the executive director to the New York Democratic Party. "Otherwise you leave yourself vulnerable to the right in both elections."

Publicly, Warren and Sanders aren't doing anything that would make people see them as rivals - albeit friendly ones.

Both Warren's and Sanders's offices declined to comment for this story.

Democrats say Warren has gained favor within the ranks of the Democratic Party.

On July 12, she was the only senator to attend a Democratic Congressional Campaign Committee breakfast at the committee's Capitol Hill headquarters.

At the breakfast, before a room of 100 people, House Minority Leader Nancy Pelosi (D-Calif.) sang Warren's praises.

She said it was rare for a senator to join the fundraising effort for the House, but that Warren had been doing everything in her power to get Democrats elected in the midterms, according to sources in the room.

"This is not typical," Pelosi said, according to a source in the room. "But that's because she's done more than anyone else. She's dedicated to winning."

Sanders, who put off some Democrats who thought he stayed in the 2016 primary too long against Clinton, also has spent considerable time since that election crisscrossing the country to campaign - and win loyalty - from fellow Democrats. Just last month, he appeared in Kansas alongside Alexandria Ocasio-Cortez, the 28-year-old democratic socialist who defeated incumbent Rep. Joseph Crowley in a primary for New York's 14th Congressional District.

A few Democrats say they do think there's room for both of the powerful progressives in a 2020 race.

"After the last election cycle, we need to have a vigorous debate, and they're two articulate leaders who deserve to fight it out in the primary," said Democratic strategist Jim Manley, who added that he's "convinced both are going to run, and I don't see one bowing out for the other. They're going to have to battle it out over who is going to carry the [left] wing of the party and I can only assume both are eying each other warily."

"We're heading for a big debate, and I for one have no problem with it," Manley said.

Another Sanders ally, requesting anonymity to speak candidly, said Sanders and Warren will figure it out as 2020 draws closer.

"The nature of presidential campaigns is that they're very individualistic and they'll decide on their own what they'll each do at the end of the day," the ally said. "On the plus side, they are two amazing candidates talking about a progressive future. But one big minus is that it could enable a third person to do much better."