Senator Cory Booker gave a pep talk to donors at a Manhattan barbecue restaurant on Wednesday evening, declaring that he was close to qualifying for the Democratic primary debate in December. But Mr. Booker acknowledged uncertain times ahead in the race, according to two people in attendance: Every time he turned around, Mr. Booker said, another candidate was ready to jump in.
There was a serious subtext in Mr. Booker’s lighthearted comment: The sudden entry of two experienced candidates, Deval Patrick and Michael R. Bloomberg, could pose risks to the Democrats who have been running for months, affecting underdogs like Mr. Booker and front-runners like Joseph R. Biden Jr., the former vice president.
Mr. Bloomberg, a billionaire, is poised to swamp the field with television and digital advertising, expanding the map of contested primaries and making it even harder for all but the highest-profile candidates to break through. And Mr. Patrick has made clear that he aims to compete fiercely on several vital fronts: In New Hampshire, where Mr. Biden is locked in a tight race with Senators Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont; in South Carolina, where Mr. Biden is relying on the overwhelming support of black voters; and in the board rooms and salons where Mr. Biden is battling for financial donations with Mayor Pete Buttigieg of South Bend, Ind.
Mr. Patrick, the former governor of Massachusetts, and Mr. Bloomberg, the former mayor of New York City, are both determined to displace Mr. Biden as the favorite candidate among voters closer to the political center, and ultimately to overcome populist champions like Ms. Warren and Mr. Sanders. It is a wildly ambitious mission, given the stature of the top candidates and the resilience of their political support so far.
Though they face long odds, Mr. Patrick and Mr. Bloomberg have already unnerved the other candidates in a race that has reached a hazy equilibrium, with Mr. Biden leading in the national polls, usually followed by Ms. Warren, and with both of them knotted up in the earliest-voting states with Mr. Sanders and Mr. Buttigieg. Even a modest shift in the early states could have an outsized impact on the national race.
The leading candidates have reacted to the changing field in different ways: Mr. Buttigieg, who has been rising rapidly as a moderate alternative to Mr. Biden, has been muted in public but has told associates privately that he believes the energy of the Democratic Party is outside Mr. Bloomberg’s areas of strength. Ms. Warren and Mr. Sanders have reveled in blasting Mr. Bloomberg as an oligarch seeking to buy the presidency, while avoiding conflict with Mr. Patrick.
And Mr. Biden has said little about the new contestants, as his advisers have quietly urged political donors not to overestimate their impact.
It remains to be seen whether the two men will inspire and animate a powerful moderate coalition in a way that Mr. Biden has not, or splinter the party’s less-liberal constituencies and effectively strengthen Ms. Warren or Mr. Sanders. Or, they might just find themselves entirely superfluous in an already crowded field.
“We have several moderates in the campaign now, but they’ve not been able to fill the lane,” said Marlon Kimpson, a state senator in South Carolina who has been hosting town hall-style events with presidential candidates. “Now, you’ve got all these people wanting to dip their toe in the water and test the likelihood that they can fill this role.”
Mr. Kimpson said Mr. Biden was still the most natural option for moderate voters, but warned that voters “don’t seem to be very motivated about his candidacy.” He said he was skeptical that either Mr. Bloomberg or Mr. Patrick could do better with such a delayed start, though he considered Mr. Patrick as by far the more intriguing option.
“I think Deval Patrick makes an excellent candidate, however, I am very, very concerned that he simply will not have enough time to make the case in the state of South Carolina,” Mr. Kimpson said, adding of Mr. Bloomberg, “We are familiar with him, but there doesn’t appear to me to be any excitement about his potential.”
Among the other Democratic campaigns, there is more confusion than consensus about what effect Mr. Patrick and Mr. Bloomberg might have on the primary, beyond straining the fund-raising efforts of other candidates who appeal to the business community. That could be an acute challenge for Mr. Biden, the only leading candidate who has been facing a significant cash crunch in recent months, as well as all of the less-prominent candidates hoping for a late breakthrough.
Neither of the two newcomers has done anything to build an operation in the early primary states, and Mr. Bloomberg has indicated he plans to skip those four February contests entirely. Any major effect they will have on the nomination fight would likely come in the states that vote after Iowa and New Hampshire, where the race is less defined.
Roxanne Conlin, a prominent Democratic lawyer in Iowa who is supporting Senator Amy Klobuchar of Minnesota, said on Thursday she saw no opening for new candidates there. Sounding very much like the kind of voter Mr. Bloomberg or Mr. Patrick would need to win over, Ms. Conlin said she had chosen Ms. Klobuchar for her mainstream, Midwestern profile, and because she could “make mincemeat out of Trump in the debates.” And, she said, she was satisfied with that choice.
“It is at least possible that some of these late entrants are driven by dissatisfaction with the current crop,” Ms. Conlin said. “That’s misplaced.”
“I’m sure that those who are entering think they have something special to offer,” she added. “But I don’t see it. I don’t.”
There is a level of frustration across the Democratic field that two candidates who have done nothing so far to build support in the traditional primary states, and who boast only modest popular followings, are garnering so much interest from donors and the media. After all, the race already has a number of candidates, including Mr. Booker, Ms. Klobuchar and Senator Kamala Harris of California, attempting to navigate the early primaries and caucuses by building a broad coalition that is ideologically moderate, racially diverse or both.
John Hagner, a Democratic political strategist, said no recent opinion surveys had suggested Democratic primary voters were desperate for new options in the race. The new candidates seemed to offer an exciting sense of novelty to political elites, he said, but the existing field was largely “more impressive” than its newest members.
“There’s no evidence in any of the polling that anyone is dissatisfied with the field,” Mr. Hagner said, adding, “Across the board, we’ve got really impressive folks that haven’t broken out yet.”
Still, even campaigns that take a dim view of Mr. Bloomberg and Mr. Patrick’s overall prospects worry about their potential to disrupt the race. Mr. Bloomberg can spend millions from his personal fortune to contest colossal primary targets like California and Texas. And Mr. Patrick, whose wide network of close relationships with alumni of the Obama administration, and a symbolically potent personal friendship with former President Barack Obama, might peel away centrist voters in New Hampshire and African-Americans in South Carolina.
So far, however, African-American voters have not abandoned Mr. Biden for either of the black candidates already competing, Mr. Booker and Ms. Harris. According to polls, the younger African-Americans who are rejecting Mr. Biden are largely doing so in favor of Mr. Sanders or Ms. Warren.
For now, allies of both Ms. Warren and Mr. Sanders believe there is little downside for either of them in the incipient Bloomberg and Patrick campaigns. Ms. Warren and Mr. Sanders have sworn off support from big donors, making them immune to financial competition from Mr. Bloomberg and Mr. Patrick, and their electoral support is largely drawn from younger people, liberals and, in Ms. Warren’s case, women — groups that Mr. Patrick and Mr. Bloomberg would likely struggle to wrest away.
But there are signs that both men could find an opening elsewhere.
In South Carolina, Marvin Pendarvis, a state representative, said he was weighing his options after his first choice, former Representative Beto O’Rourke of Texas, dropped out. Mr. Pendarvis said he hoped to hear from Mr. Patrick, calling him “one I have always looked at with intrigue” and speculating that South Carolina’s primary could become a wide-open affair if Mr. Biden falters in earlier contests.
“It’s no secret that Joe is not doing well in Iowa,” Mr. Pendarvis said. “If that happens, does that send a message to South Carolinians, about the possibilities of his campaign?”