FAIRFIELD, Iowa — Diane Chojnowski and Denyce Rusch were among the Iowans who braved light snowfall and temperatures in the teens to see Senator Bernie Sanders on Sunday afternoon, a few hours before Senator Elizabeth Warren was also due in this liberal pillar of eastern Iowa.
But after Ms. Chojnowski and Ms. Rusch praised Ms. Sanders, they turned to a predicament far more bothersome than the winter weather: choosing between the two progressive candidates.
“You worry about Bernie and Elizabeth splitting the progressive vote because between the two of them they’ve got a huge bloc,” said Ms. Chojnowski, who initially considered Ms. Warren but has come back to Mr. Sanders because, as she put it, “he’s the thought leader.”
She is not the only liberal voter alarmed.
Since the presidential primary race began, the two progressive senators — who have been friends since 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 progressives divide votes in so many contests that neither is able to capture the nomination. Moderates in the party fear that if Mr. 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.
Interviews with aides from both camps — who spoke on the condition they not be named because they warn their own surrogates not to criticize the other — produce a common refrain. The two candidates are loath to attack each other because they fear negativity would merely antagonize the other’s supporters. The only way to eventually poach the other’s voters, each campaign believes, is by winning considerably more votes in the first caucuses and primaries.
Liberal leaders, acknowledging the mixed blessing of having two well-funded, well-organized progressive Democrats dividing endorsements and poised to compete deep into the primary calendar, are now beginning discussions about how best to avert a collision that could tip the nomination to a more centrist candidate.
At informal Washington dinners, on the floor of the House and on activist-filled conference calls, left-leaning officials are deliberating about how to forge an eventual alliance between Mr. Sanders, of Vermont, and Ms. Warren, of Massachusetts. Some are urging them to form a unity ticket, others want each to stay in the race through the primary season to amass a combined “progressive majority” of delegates, and nearly every liberal leader is hoping the two septuagenarian senators and their supporters avoid criticizing each other and dividing the movement.
“It can’t be, One candidate is the true god or goddess and the others are just shills,” said Representative Pramila Jayapal of Washington, who backed Mr. Sanders in 2016 but has not yet endorsed anyone in this race. “It really has to be, We’re trying to strengthen them both.”
On Saturday, Robert Reich, a former secretary of labor, sent an email to members of the progressive group Democracy for America warning that if supporters of the two candidates “wage war on each other” that it would “take both of them down.”
Former Senator Harry Reid of Nevada, one of the few prominent Democrats who commands the respect of both Mr. Sanders and Ms. Warren, said he was ready to step in if needed.
“I am standing by,” said Mr. Reid, the longtime Senate leader. “And if I see an opportunity that I can broker some kind of a deal, I’ll try.”
That progressives find themselves in this situation at all is extraordinary. Perhaps not since anti-establishment Democrats were choosing between two other left-leaning senators, Robert F. Kennedy and Eugene McCarthy, in 1968, have there been two such liberal titans in the same primary contest.
Mr. Sanders and Ms. Warren have capitalized on the leftward drift of many younger Democrats and the weakening of the party’s traditional gatekeepers. Indeed, Mr. Sanders’s surprising success in the 2016 presidential primary race represented something new: a left-wing hopeful with the money and the organization to sustain a long campaign.
“Jesse Jackson was raising $5 at a time in church basements,” Mike Lux, a longtime liberal organizer, recalled of the civil rights leader’s two White House bids in the 1980s. “But now online donors have changed everything.”
With both Mr. Sanders and Ms. Warren stockpiling over $25 million going into next month, and both enjoying a reliable stream of low-dollar contributors, they won’t soon suffer the most common cause of political death in a primary election — running out of money.
Yet this shared strength has only further confounded progressives who are hung up on the same question: How does it end?
Representative Ro Khanna of California, a co-chairman of Mr. Sanders’s campaign, said the best solution would be to create a Sanders-Warren ticket.
“The two of them could usher in a progressive era for the next decade,” said Mr. Khanna, likening such a maneuver to what Bill Clinton did by tapping another young and fairly moderate Southerner, Al Gore, as his running mate in 1992. “They doubled down on a bet for a centrist vision of the party. This would be a bet on a progressive vision for the party.”
Another Sanders supporter, Representative Ilhan Omar of Minnesota, was equally effusive. “I think that would just be the dream of all progressives,” she said. “When you’re going into a battlefield, you want your best players to be on the field starting. And they are our best.”
Centrist Democrats, of course, think just the opposite. Mr. Biden, the former vice president, even after facing intense media scrutiny and racking up a number of self-inflicted errors, still fares slightly better in head-to-head polling against Mr. Trump than either of the two progressive front-runners.
Yet it was the combined strength of Ms. Warren and Mr. Sanders, along with Mr. Biden’s fund-raising and debate difficulties, that frightened moderates enough to lure former Mayor Michael R. Bloomberg of New York and former Gov. Deval Patrick of Massachusetts into the race. Now, though, it’s the left wing of the party that’s growing anxious about how to avoid its own debacle.
A handful of leaders from progressive labor unions have had initial conversations about if, when and how to collectively endorse a candidate and are planning to meet after the holidays, according to a Democrat familiar with the discussions.
Larry Cohen, the former president of the Communications Workers of America, is already thinking well ahead of that. Mr. Cohen supports Mr. Sanders but that prefers Ms. Warren stay in the race throughout the primary election to deny a moderate from accumulating a delegate majority. If Mr. Sanders doesn’t win outright, he said, the two progressives should demonstrate their combined strength, and amass over half the pledged delegates, so that the left has an upper hand going into next summer’s Democratic convention.
Mr. Cohen has been more aggressively making his case to other progressives about how both candidates should stockpile delegates, including over dinner last week with other Sanders supporters.
But in the near term, he and other left-wing organizers are mostly trying to keep the primary-within-a-primary between Mr. Sanders and Ms. Warren from growing contentious.
On a conference call last month organized by Our Revolution, the Sanders-aligned progressive group, Ms. Jayapal and Mr. Cohen both urged activists to exert pressure on House Democrats to support “Medicare for all” while highlighting the common ground of the two presidential hopefuls.
“We’ve got two candidates who are all in for Medicare for all, Bernie Sanders and Elizabeth Warren,” Ms. Jayapal said on the call. “There are differences in approach. But, at the end of the day, both of them are committing to get the transition to Medicare for all done in four years.”
It’s Ms. Warren’s new posture on that issue, though, that has halted her momentum — and it’s health care that could unravel her and Mr. Sanders’s nonaggression pact. Until she unveiled her own, more gradual implementation plan — after weeks of saying “I’m with Bernie” on Medicare for all — Ms. Warren had carefully guarded her left flank to avoid handing Mr. Sanders any fodder.
Now, Mr. Sanders’s aides say Ms. Warren’s shift has signaled to progressive voters that she is malleable on what they consider a fundamental values issue. Her pivot on health care, along with the support she enjoys from a handful of billionaires, represents the best opportunity to diminish liberal enthusiasm for Ms. Warren, say Sanders supporters.
Ms. Warren’s enthusiasts say Mr. Sanders will prove unable to unite the entirety of the party he has long resisted formally joining.
“Obviously it would help if he were not in the race,” Representative Jan Schakowsky of Illinois said. “But I think she can get her votes from a lot of different places.”
Privately, though, many supporters of Ms. Warren worry that Mr. Sanders will never drop out. And some fear his persistence in the race will damage her.
“Sanders is a problem for her in two ways: one in terms of the votes that would otherwise go to her, and two, by forcing her to go to the left,” former Representative Barney Frank of Massachusetts said. “Her ability to respond to concerns about electability is hampered by her concern that she’ll be overtaken by Sanders on the left.”
For all their public praise for each other, Ms. Warren and Mr. Sanders are not in frequent contact. They have not explicitly discussed any hands-off agreement, let alone considered any future unity effort. The last time they spoke, aides said, was a brief conversation last month in which Ms. Warren told him about her new health care plan and he politely noted he had a different view.
But with 50 days until the Iowa caucuses, some of their admirers are hoping they will begin a dialogue.