Democrats’ pledge to reject corporate-PAC money is mostly symbolic.

Now, four years later, as the 76-year-old Sanders positions himself toward a second run for president, it’s Warren who again looms largest over his designs. At a January strategy meeting at the Washington, D.C., apartment of the aide Ari Rabin-Havt, Sanders acknowledged to confidantes in the room that the biggest threat to his pursuit of the 2020 nomination would be the 69-year-old former Harvard Law professor, according to a person familiar with the discussion.

As the two most recognizable faces of the progressive movement, Sanders and Warren are natural allies on a host of liberal causes, none more so than the economic inequalities that strain the nation. And yet each side’s camp believes that when it comes to the next presidential contest, the Democratic primary is only big enough for one of them.

Advisers to both senators tell me they strongly doubt the two ideological allies would run against each other, given their genuine mutual fondness and the sober realization that their support bases vastly overlap. To do so would likely mean mutually assured destruction.

So which one goes? And how will they decide?

The ultimate resolution of this progressive predicament will be one of the most crucial factors in the fast-approaching battle on the left to topple President Donald Trump. The gathering threat of Sanders and Warren doesn’t seem lost on the president. In an echo of his approach to his competitors in 2016, he’s rebranded them with the disparaging monikers “Crazy Bernie” and “Pocahontas.” Recently, Trump has openly fantasized about debating Warren and presenting her with a genetic-testing kit to prove her claimed Native American heritage.

At the same time, allies of Warren and Sanders are beginning to privately pick apart their would-be competitor’s political pedigree, casting their favorite as the far superior choice to go up against the president. At this early stage of the race, their comments could be seen as mere posturing. But they are nevertheless revealing, exposing fault lines between the two lawmakers that could soon spill into public view.

Sanders advisers and allies believe he’s earned the right of first refusal: He was the runner-up to Clinton, galvanized a fresh flock of young voters, and fundamentally reframed the issue matrix for a hyper-progressive party going forward. He would start with a leg up: a nationally tested organization with the hardened experience of one presidential run already under its belt, an email list of 7 million proven donors, and the ability to raise hundreds of millions of dollars. (Tim Tagaris, Sanders’s digital–fund-raising guru, has privately floated a range of between $275 million and $300 million for a primary campaign, one aide recalls.) A common refrain bandied about in Bernieland is that Sanders won at least 40 percent of the primary vote in 37 states.