Month after month, former Vice President Joseph R. Biden Jr. led national polls in the Democratic presidential primary race. But in early October, someone else seemed to crack the code. Lifted by her bevy of policy plans and grass-roots momentum, Senator Elizabeth Warren of Massachusetts joined Mr. Biden in a statistical tie atop the party’s large field of candidates.
It didn’t last.
What happened over the following days would reset the trajectory of the primary race: Mr. Biden stabilized; Ms. Warren and Senator Bernie Sanders of Vermont began experiencing a reversal of fortunes; Mayor Pete Buttigieg of South Bend, Ind., drew a bead on Ms. Warren; and Senator Kamala Harris of California looked less and less like a formidable candidate.
That week in mid-October — the run-up to the Oct. 15 Democratic debate and afterward — encapsulated the fluid and uncertain nature of the biggest presidential field ever. And it can function as a cheat sheet for voters trying to understand the state of the race heading into 2020.
“Candidates being under attack, facing scrutiny or going on offense themselves — that week had all that going on and is a good explanation for how we’ve gotten to this point in the primary today,” said Waleed Shahid of Justice Democrats, the progressive group advocating for left-wing candidates in Congress and in the primary.
Among the turning points was the debate itself: The surging Ms. Warren experienced significant pressure to clarify her position on “Medicare for all,” which culminated in her releasing financing and transition plans on health care that have since ignited some blowback. She has not recovered from her polling peak since.
Mr. Sanders, who had suffered a heart attack at the beginning of the month, bounced back with a lauded debate performance, and his campaign announced endorsements from Representatives Alexandria Ocasio-Cortez of New York and Ilhan Omar of Minnesota on the night of the Oct. 15 debate. Even since, the party’s left-wing has continued to rally around Mr. Sanders’s candidacy.
“Because there was so much chatter about how the Sanders campaign was over, they got to surprise everyone. And that doesn’t happen all the time in politics,” said Rebecca Katz, a progressive consultant in New York City. “And he’s been rolling since then.”
Mr. Buttigieg, in turn, helped change the tone of a markedly cordial Democratic race. He first accused Ms. Warren of “evasiveness” on health care in September, but during a Wall Street Journal interview published Oct. 13, he ratcheted up the attack, addressing Ms. Warren directly.
“I think you should be straightforward about what your plan is going to do,” Mr. Buttigieg said, “and we haven’t seen that.”
It was during this time that he crystallized a hard pivot to the party’s ideological center, which ultimately helped him become a top-tier candidate in the early voting states of Iowa and New Hampshire.
It also enraged liberals, who continue to call Mr. Buttigieg inauthentic.
“The biggest long-term impact of that past chapter of the campaign is that Pete permanently destroyed his brand,” said Adam Green, a co-founder of the Progressive Change Campaign Committee, which is supporting Ms. Warren. “He went from being the authentic, unobjectionable, clean-cut guy to being seen as mean, dishonest, and in the pocket of Big Insurance and other big-money interests that attend his fund-raisers.”
Mr. Biden escaped the October debate without sustained attacks from his opponents. It reflects a notable trend that has continued through the end of the year: Ms. Warren, Mr. Sanders, Ms. Buttigieg and Senator Amy Klobuchar of Minnesota have focused their attacks on each other, even as Mr. Biden continues to be the front-runner.
“There is a reticence to attack him because it feels like there’s something you’ll have to pay for in the general,” said Akunna Cook, the former executive director of the Black Economic Alliance. “I think people have started to presume that he’s the nominee, people are starting to think it’s inevitable.”
“The progressives are so focused on Bernie versus Elizabeth that they’re not focused on Biden,” she added.
Mr. Shahid said that the former vice president has stayed away “from the level of scrutiny he received during the first months of his campaign.”
Mr. Biden’s permanence is real — his lead atop national polling averages has gone virtually unthreatened since Ms. Warren receded from her peak. It has defied the expectations of some party leaders who began the year boasting about the historic diversity of the Democratic field, which included several female candidates and black and Latino candidates. But in the December debate, six of the seven candidates onstage were white, and only one woman — Ms. Warren — remains in the top tier.
The campaign will now enter the new year almost exactly where it began — with Mr. Biden as the presumed front-runner and Mr. Sanders as his closest rival.
In the days before the October debate, candidates also revealed their third-quarter fund-raising totals — the first clear signs that the race was trending toward an all-white top tier. It highlighted the failure of the race’s two leading black candidates at the time — Ms. Harris and Senator Cory Booker of New Jersey — to gain significant traction, hurt by their low poll numbers among black voters.
In the run up to the debate, Mr. Sanders, Ms. Warren and Mr. Buttigieg announced raising more than $19 million in the third quarter of fund-raising — while Ms. Harris was the only nonwhite candidate to crack double digits.
On Oct. 12, Ms. Harris held a block party in Des Moines, part of her campaign’s push to hold an event in Iowa every week in October. Her late focus on Iowa was one of a series of ill-fated strategic moves, which culminated in her surprise exit from the race in November.
At the debate days later, Ms. Harris tried to goad Ms. Warren into an argument about whether to delete President Trump’s Twitter account, which the Massachusetts senator laughed off. It was a sign of Ms. Harris’s diminished standing that Ms. Warren and her campaign team did not feel the need to respond.
Also in early October, the impeachment inquiry led by House Democrats began taking closed-door depositions. Their timeline set in motion dynamics that now threaten to upend the nomination process.
President Trump, for one, has already promised to make next year’s re-election in part a referendum on House Democrats. And a Senate impeachment trial during the Democratic primaries and caucuses could complicate the campaign schedules for the crop of senators still in the race — Mr. Sanders, Ms. Warren, Ms. Klobuchar, Mr. Booker and Senator Michael Bennet of Colorado.
“I swore an oath to protect and defend the Constitution,” Mr. Booker has said. “I didn’t swear an oath to protect and defend the Constitution unless there’s an election coming up.”
Impeachment also impacted the Biden campaign. In late September and early October, Mr., Biden struggled to combat President Trump’s attacks against his son, Hunter Biden, and his business dealings in Ukraine. By mid-October, Mr. Biden had begun hitting back more effectively on Mr. Trump’s pressure campaign on Ukraine to investigate the Bidens. And on Oct. 15, ABC News broadcast an interview with Hunter Biden in which he admitted fault but also defended himself.
“I gave a hook to some very unethical people to act in illegal ways to try to do some harm to my father. That’s where I made the mistake,” the younger Mr. Biden said. “So I take full responsibility for that. Did I do anything improper? No, not in any way. Not in any way whatsoever.”
Every race is fluid, of course, and the dynamics sets in motion in mid-October could shift, particularly considering that the Iowa caucuses famously break late. In the Dec. 19 debate in Los Angeles, for example, Ms. Warren departed from her traditional refusal to criticize other candidates — as did Ms. Klobuchar, who directly chided Mr. Buttigieg.
One big unknown is fund-raising. On Oct. 16, Mr. Biden disclosed a cash-on-hand number so low — $9 million — that it startled many in the party’s moderate wing and some supporters. His apparent vulnerability as a front-runner helped spur two new entrants in the race, former Gov. Deval Patrick of Massachusetts and Michael R. Bloomberg, the former New York City mayor and one of the richest people in the country.
“When Biden showed, not that just that fund-raising was lackluster, but that they were spending more money than they were raising, it showed not just that the candidate was in trouble,” Ms. Katz said, “You had a campaign team that was committing malpractice.”
One of the big questions of the Democratic race now is whether Mr. Bloomberg’s self-funded candidacy can break through to moderate voters, or whether they will coalesce around Mr. Biden, Mr. Buttigieg or another contender. Mr. Biden’s fund-raising and poll numbers largely stabilized through the fall, and he begins 2020 in arguably his strongest shape since things started turning around in that week in October.