John Bazemore / AP

Near the end of the latest Democratic debate, Cory Booker did something unusual for a presidential candidate: He admitted that his campaign was in trouble. “I have not yet qualified for the December stage,” the senator from New Jersey confessed, “If you believe in my voice and that I should be up here, please go to CoryBooker.com. Please help.”

The plea worked. A surge of contributions pushed Booker past the donor threshold needed to quality for the next debate, which is scheduled for December 19. But that might not be enough. To take the stage, Booker also needs at least 4 percent in four national or state polls or at least 6 percent in two polls from early states. As of November 24, according to The New York Times, he doesn’t have a single one. If that doesn’t change between now and the cutoff date of December 12, Booker’s campaign will be left for dead.

How has a candidate whom CNN last December placed fourth in its power rankings—ahead of Elizabeth Warren and Bernie Sanders, not to mention Pete Buttigieg, who didn’t even make the list—found himself trailing Tulsi Gabbard, Tom Steyer, and Andrew Yang in both Iowa and New Hampshire? The answer has a lot to do with Booker’s unwillingness to stand up for what he once believed. Since early this year, Democratic moderates who are uneasy about Joe Biden have been casting about for a candidate. But Booker, by refusing to challenge his party’s left in the early debates, took himself out of contention. And now it may be too late.

In his early political career, Booker embodied the market-friendly, fiscally conservative ethos of Bill Clinton’s Democratic Party. In 2002, when he ran for mayor of Newark as a darling of Wall Street who supported school vouchers, New York magazine called him “essentially a Clinton Democrat.” Jesse Jackson dubbed him “a wolf in sheep’s clothing.” In 2011, Booker enraged New Jersey’s public-employee unions by backing Governor Chris Christie’s effort to cut health and retirement benefits for teachers and other state workers. And the following year, Booker—who during the 2012 election cycle received more than one-third of his campaign contributions from the finance industry—famously called on Barack Obama’s campaign to “stop attacking private equity” in an interview on Meet the Press.

Booker’s defining decision as a presidential candidate has been his refusal to defend this centrist, pro-business record. Since the campaign began in earnest this summer, its master narrative has been the clash between Bernie Sanders and Elizabeth Warren, who call for radically expanding the federal government’s role in the economy, and a rotating cast of relative moderates who attack such efforts as fiscally and politically ruinous. Again and again, despite numerous invitations, Booker has refused to join the attack.

In the first presidential debate, in June, Senator Amy Klobuchar and Representative John Delaney condemned Warren’s call to end private health insurance. But when moderator Lester Holt offered Booker the chance to join the fray, he allied with Warren and declared himself in favor of Medicare for All. When NBC’s Savannah Guthrie asked Booker to explain why he opposed Warren’s “plan to break up tech companies like Facebook, Amazon, and Google,” Booker replied, “I don’t think I disagree.” Puzzled, Guthrie noted, “You did say that you didn’t think it was right to name names, to name companies and single them out as Senator Warren has.” But again, Booker would not take the bait. “I will single out companies like Halliburton or Amazon that pay nothing in taxes,” he insisted.

At the second debate, in July, it was more of the same. After Biden and Kamala Harris sparred over the future of private health insurance, CNN’s Dana Bash asked Booker whether Medicare for All would “pull private health insurance from more than 150 million Americans.” But instead of answering the question, Booker criticized his fellow candidates for not focusing their attacks on Donald Trump. “This pitting progressives against moderates, saying one is unrealistic and the other doesn’t care enough,” he charged, “is dividing our party and demoralizing us in the face of the real enemy here.” When Bash asked Booker whether “the job guarantee in the Green New Deal”—a key element of the party’s leftward lurch—was “realistic,” Booker again ducked the question, and criticized Trump for leaving the Paris climate treaty. When Biden and Senator Michael Bennet challenged another new progressive orthodoxy—Julián Castro’s call to decriminalize undocumented border crossings—Booker chided them for “playing into Republican hands” by “trying to divide us against each other.”

Booker’s can’t-we-all-get-along retreat from his Clintonian past continued in the third debate, in September. Asked again about scrapping private health insurance, he replied, “I believe in Medicare for All” and then suggested that the debate didn’t really matter, because “every one of my colleagues on this stage is in favor of universal health coverage and comes at this with the best of intentions.”

It’s easy to see why Booker adopted this tack. Conventional wisdom holds that candidates who go negative hurt themselves even when they draw blood. Moreover, during the summer and early fall, Warren rose from the pack to draw even with Biden nationally and surpass him in Iowa, thus confirming the widespread perception that Democratic voters were hungering for an ambitious, unapologetic progressive.

But as Warren rose, so did a backlash among Democratic donors and officials who saw her economic policies as dangerously radical and feared that she could not defeat Trump. As Biden careened from poor performance to poor performance, they began searching in earnest for an alternative. Booker might have been it. He was better known to party and financial elites than centrists such as Klobuchar, Bennet, and Montana Governor Steve Bullock; possessed more star power; and offered a greater chance of putting together the coalition of black and relatively moderate white voters that usually powers successful Democratic presidential campaigns. Unlike Kamala Harris, another African American senator who is more moderate than Sanders and Warren, Booker also had a long record of making the very arguments for a pro-business, deficit-conscious liberalism that the Democratic elites who feared Warren and Sanders craved.

Yet Booker refused to play that role. And that refusal helped Pete Buttigieg step into the breach. Initially, Buttigieg’s rise was powered less by ideology than by persona. His first breakthrough came during a CNN town-hall appearance in March. By spring, his novel combination of talents and attributes—gay, veteran, religious, midwestern, hyper-educated, dazzlingly articulate—was attracting comparisons to another political unicorn, Barack Obama. But after rising into the mid-single digits in national polls in April, Buttigieg plateaued through the summer and into the fall. As late as September, he still trailed Biden, Warren, and Sanders in both Iowa and New Hampshire by double digits.

Buttigieg’s initial breakthrough came less with voters than with donors. After initially making inroads among LGBTQ contributors, he emerged over the summer as the undisputed darling of the very money people who might have been Booker’s natural base of support. In June, a New York Times headline announced that “Wall Street Donors Are Swooning for Mayor Pete.” Buttigieg wasn’t yet attacking Sanders and Warren’s left-populist agenda. But by embracing big-donor fundraisers, which they eschewed, he signaled that he’d give pro-business Democrats a seat at the table. And in the absence of a compelling, avowedly centrist candidate, that was enough. As one finance-industry lobbyist told The Washington Post, “I don’t think Wall Street has a real great sense of what Mayor Pete might mean or not mean. But we do know what Warren means, and so I want part of every story about her to also be about him moving in tandem up the line.”

By summer, Buttigieg had established a massive lead among donors. After outraising Booker by $2.4 million in the first quarter of this year, he outraised him by more than $20 million in the second. Buttigieg used the money to build a vast on-the-ground operation in Iowa. And he used it to develop a message that solidified his position as Sanders and Warren’s moderate competitor. After spending more on polling from July to September than any other Democratic candidate except Tom Steyer, Buttigieg began crafting the centrist critique that Booker had failed to articulate. After Labor Day, he unveiled an ad that promised “real solutions, not more polarization.” Another declared that while “others say it’s Medicare for All or nothing. I approved this message to say the choice should be yours.” At the fourth Democratic debate, in October, Buttigieg slammed Warren for failing “to explain how a multitrillion-dollar hole in this Medicare for All plan that Senator Warren is putting forward is supposed to get filled.” He attacked Beto O’Rourke’s call for mandatory gun buybacks as unrealistic and an example of ideological “purity tests.” But when CNN’s Anderson Cooper asked Booker to arbitrate the fight, he offered more kumbaya: “I again worry about how we talk to each other and about each other.”

Buttigieg wasn’t merely parroting his big-money donors’ views. He didn’t challenge Warren’s wealth tax, which enjoys strong popular support. But by challenging Medicare for All, which is far less popular, and attacking Warren for being dogmatic and polarizing—thus playing on the anxieties of voters who doubt she can beat Trump—Buttigieg crafted a centrist message that appealed to donors and voters alike. From mid-September to mid-November, he rocketed from fifth to first place in Iowa. While Warren still excels among liberals, Buttigieg—according to an analysis by FiveThirtyEight—has assembled a coalition that includes more moderates.

Belatedly, Booker is now trying to carve out his own center lane. After downplaying his support for charter schools early in the campaign, Booker last week wrote a New York Times op-ed championing them and attacking other Democrats for their tendency to “dismiss good ideas because they don’t fit into neat ideological boxes.” In last week’s debate he noted, “I don't agree with the wealth tax, the way that Elizabeth Warren puts it,” and suggested that “we Democrats also have to talk about how to grow wealth” rather than just how to tax it—a classic Clintonian line.

But it may be too late. Booker is low on money; in the third quarter of this year he spent more than he raised. He’s running 11th in national polls. In theory, his appeal to black voters (among whom Buttigieg struggles) and his record of market-friendly progressivism, especially in Newark, where charter schools did help improve public education, make him a stronger moderate alternative to Warren and Sanders than the South Bend mayor. But at this late date in the primary race, the centrist case for Booker remains mostly theoretical because he has barely made it himself.

In his 2006 book, The Audacity of Hope, Obama—to whom Booker has often been compared—served up Clintonian mantras like “I believe in the free market, competition, and entrepreneurship, and think no small number of government programs don’t work as advertised” and “I reject a politics that is based solely on racial identity, gender identity, sexual orientation, or victimhood generally.” Perhaps no Democratic candidate could win a primary talking that way today. Maybe that’s a good thing. But it’s notable that Buttigieg’s rise in the polls has coincided with his willingness to tack, gingerly, back in Obama’s direction.

What would have happened had Booker—who, like Obama, has a history of transgressing liberal orthodoxy—challenged his party’s leftward shift from the beginning of his presidential campaign? There’s no guarantee that he would have done better. But the harsh reality is that he could hardly have done worse.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.