"Since the presidential primary race began, the two senators — who have been friends since before Ms. Warren was elected to the Senate in 2012 — have abided by a de facto nonaggression pact, rarely criticizing one another and frequently acting as something of a populist tag team on the debate stage. And as Pete Buttigieg has risen in the polls and Joseph R. Biden Jr. has proved durable, Mr. Sanders and Ms. Warren have been happy to demonstrate their left-wing bona fides by contrasting themselves with the more moderate contenders. Yet with Mr. Sanders enjoying a revival after his heart attack in October and Ms. Warren receding from her summer surge but wielding a formidable political organization in the first nominating states, it’s increasingly clear that their biggest obstacle to winning the Democratic nomination is each other. In Iowa and nationwide, they are the leading second-choice pick of the other’s supporters, a vivid illustration of the promise and the peril that progressives face going into 2020: After decades of losing intraparty battles, this race may represent their best chance to seize control from establishment-aligned Democrats, yet that is unlikely to happen so long as Ms. Warren and Mr. Sanders are blocking each other from consolidating the left. For center-left Democrats, that’s exactly their hope — that the two senators divide votes in so many contests that neither is able to capture the nomination. Moderates in the party fear that if Ms. Warren or Mr. Sanders pull away — or if they ultimately join forces — the ticket would unnerve independent voters and go down in defeat against President Trump."