NEWTON, Iowa — Joseph R. Biden Jr. has built an early lead in the month since he entered the presidential campaign, confidently projecting himself as the Democratic front-runner and the candidate best positioned to defeat President Trump.
But beneath the surface of a seemingly placid race is a much more volatile contest, as a series of primaries-within-the-primary unfold along lines that reflect some of the most animating forces in the Trump era: race, gender, age and ideology.
With a historically large field of 23 candidates apparently now set, Senators Kamala Harris and Cory Booker, both African-American, are competing with Mr. Biden and other candidates for the support of black voters; Pete Buttigieg and Beto O’Rourke, who are both under 50, are vying for the mantle of generational change; Senator Elizabeth Warren is encroaching on Senator Bernie Sanders’s support from the party’s left wing; and six women are making the case that it is long past time for a female president.
As the first debates loom next month and candidates begin to test initial attacks on Mr. Biden, and one another, these miniature races will help clarify who might emerge as the most formidable alternative to the former vice president. As Mr. Trump demonstrated in 2016, initial assumptions about who primary voters will and will not support can prove foolhardy.
“It’s easy for those of us who are deeply involved in the process to get so wrapped up in it that we forget just how many people will tune in for this very late in the calendar year,” said Mr. Buttigieg, the mayor of South Bend, Ind., who like other candidates spent the Memorial Day weekend campaigning.
Referring to Mr. Sanders, he said that other than “the more ideological candidates” nobody’s “support is consolidated at this point.”
Mr. Biden, hoping to stay above the fray of the primary, has found success ignoring his rival candidates and putting Mr. Trump at the center of his campaign. He is keeping a notably light schedule and carefully avoiding political risks. At a moment when many in his party are shaped by the post-traumatic stress lingering from 2016, Mr. Biden’s implicit message, that he can win, carries significant appeal.
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However, interviews with dozens of party leaders and strategists revealed a far more fluid race than Mr. Biden’s double-digit polling advantage would indicate, especially in the early-nominating states where so many Democratic races had been upended over the years.
“My party doesn’t like front-runners,” said Paul Begala, the former top adviser to former President Bill Clinton, noting that nearly every Democratic favorite in recent history either lost the nomination or suffered a scare along the way. “It’s going to tighten no matter what.”
Democrats say that is in part because much of the party’s energy is coming from younger, female and progressive activists, making it unlikely that a 76-year-old, establishment-aligned white male will simply march to the nomination. And while Mr. Biden has benefited so far from the sprawling field, which has split the opposition, there are already signs that he could face stiffer competition.
In the race to become the progressive alternative to Mr. Biden, multiple polls indicate that Ms. Warren is gaining on Mr. Sanders, the other septuagenarian in the race who is running second in most surveys. And while she has so far declined to take on the Vermont senator by name, Ms. Warren has repeatedly appeared together in recent days with Representative Alexandria Ocasio-Cortez, a galvanizing force among progressives, sending an unmistakable signal that she intends to pursue Mr. Sanders’s left-wing base.
There are also indications that many in the party would prefer a woman to be their standard-bearer: 31 percent of Democrats surveyed in a new Pew poll said they would be “more enthusiastic” about a female nominee, a number that rose to 45 percent among Democratic women under the age of 50. And in a new poll from Monmouth University taken after a series of state abortion bans, three women — Ms. Warren, Ms. Harris and Senator Amy Klobuchar of Minnesota — are building support.
Claire Celsi, an Iowa state senator, said she’s narrowed her choices to the women in the race.
“I see the brilliance in all the female candidates and I want to shine a little light on that,” Ms. Celsi said. “Because of what happened to Hillary Clinton I feel like I owe them that.”
She has already met privately with five of the six female candidates, had phone calls with Ms. Harris’s husband and influential sister and next month will sit at Ms. Harris’s table during a major fund-raising dinner for the Iowa Democratic Party.
Indeed, no other state better illustrates the volatility of the primary than Iowa, where the contours of the last three Democratic caucuses have changed late in the race, most notably when Barack Obama thrashed Ms. Clinton in 2008, jump-starting his rise to the nomination.
Appearing Friday at a house party in Newton, just east of Des Moines, Mr. Booker warned Iowa Democrats not to seek “a savior in this election” and said that 2020 must be about something bigger than simply unseating Mr. Trump — unmistakable references to Mr. Biden and his message.
In a brief interview after the event, Mr. Booker recalled that Mr. Trump himself had said that he alone could successfully confront the country’s challenges. “We have to resist the temptation to project upon one candidate the solving of all of our problems,” Mr. Booker said.
Mr. Biden and Mr. Sanders are knotted together above the rest of the field right now, according to public and private polls, but at this time in 2015 Ms. Clinton had led Mr. Sanders by 41 points in a Des Moines Register poll. She would eke out victory by less than a single point.
“There is a long way to go and at this point no one thought that a peanut farmer from Georgia was going to be president, or that a governor from Arkansas was going to win or that a guy named Barack Obama was going to be able to capture the White House,” Ms. Klobuchar said.
Voters like Linda Wormley make clear that it is folly to assume the two current Iowa leaders will remain as such next February.
Ms. Wormley, a retiree who attended Mr. Booker’s house party, said she had a list of six potential candidates to support, but instead of identifying them she mentioned the two contenders she had ruled out: Mr. Biden and Mr. Sanders.
“I’m open to the women and a couple of the men,” said Ms. Wormley, who invoked the House speaker to argue that a woman would have the best chance to rattle Mr. Trump. “Look at what Pelosi has done to him,” she said.
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While Mr. Biden enjoys significant good will in Iowa, organization is crucial to success in the caucuses, and he is only now building an operation while some of his rivals have already put in place a more substantial infrastructure.
“He’s going to have to pick up the organization,” said former Gov. Tom Vilsack of Iowa. “It requires a lot of people and Warren has lots of people on the ground and they’re good.”
The most engaged Iowa activists see a more wide-open race than polls indicate, with the female contenders as well as Mr. Buttigieg drawing frequent mention.
“It seems the press and perhaps the party have landed on Biden as the top contender, when Harris, Buttigieg and others are quite popular with the Democrats I know,” said Marjie Foster, the party chairwoman in Decatur County. “We are scratching our heads a bit at the polling information because it truly doesn’t seem to reflect how the majority of my sphere of influence feels.”
Sean Bagniewski, the party chairman in Polk County, which includes Des Moines and represents about 20 percent of the state’s Democrats, said the vast field was prompting many Democrats to take their time.
“Instead of having one favorite or two favorites, people have three to five favorites,” Mr. Bagniewski said. “Other than people working for the candidates, I know less than 10 people who have made up their minds.”
Perhaps no subject is touchier in the Democratic contest than the age of the two front-runners. It comes up repeatedly, if politely, in conversations with voters. And the issue was on vivid display in the new Pew poll, which asked voters the ideal age for their nominee: just three percent said somebody in their 70s and only six percent said a candidate in their 30s (Mr. Buttigieg is 37).
For now, Mr. Biden, who has vowed to restore dignity to government and forge consensus in Washington, is dominating the field among older voters.
“That’s a reaction to the fact that folks my age don’t like the tone and content from this president,” said Mr. Vilsack, who is 69. “They want some stability, they want an adult in the room who’s tough enough to beat President Trump.”
But younger voters, said Bryce Smith, the Democratic chairman in Dallas County, “are saying the wheel is totally dismantled and we need to reinvent and rebuild the wheel.”
The generational divide, Mr. Smith predicted, would be “the biggest division in my eyes leading into the caucus and primary season.”
Terry Kocher, the party chairman in Humboldt County in northwest Iowa, said he donated to Mr. Buttigieg’s campaign after being impressed with his “calmness, intelligence, poise and wit during interviews.”
“It is time for the torch to be passed,” he said. “To have someone who looks more like you, talks more like you and better understands how you feel is what will drive us to victory for years to come.”
That sentiment was evident as Mr. Booker left the home of Fritz and Carol Kramer Friday afternoon. Ms. Kramer said she was excited about him and Ms. Harris.
“A lot of my older friends really like Joe because they think he can beat Trump and they like him because they knew him,” she said. “And he is a good person — but I’m still wanting some young people and people of color.”