In the suburban enclaves just north of New York City, a seven-way race to replace a retiring matriarch in the House of Representatives has devolved into something far less idyllic: a venomous fight between the son of a pharmaceutical billionaire and rivals who accuse him of trying to use his wealth to buy political power.

The high stakes may well explain the ferocity. The race represents a once-in-three-decades opportunity to win an open seat in a secure Democratic district, where incumbency is likely to grant the winner a lengthy tenure in Congress.

More than $7 million has already been spent on the contest, and more than half of that comes from one wealthy candidate who is trying to break away from a crowded pack.

Adam Schleifer, a former federal prosecutor in California, has spent more than $4 million on the race in the district, which covers Rockland County and part of Westchester. That’s roughly $1 million more than the six other contenders combined.

Most of Mr. Schleifer’s campaign war chest comes from his own pocket — $3.7 million in total, a reflection of the wealth he derives from his father, Leonard Schleifer, whose company, Regeneron, boasts a $50 billion market capitalization. But he has received donations, too, including from real estate developers like Jeffrey Gural, a prominent Democratic fund-raiser.

Mr. Gural said that Mr. Schleifer, 38, “understands the issues,” but he acknowledged that a victory could raise prickly questions about the power of money.

“If he wins, it shows that you can buy an election,” Mr. Gural said in a recent interview.

The contest is likely to decide who succeeds Representative Nita Lowey, the first woman to chair the House Appropriations Committee and a beloved figure among Democrats in the lower Hudson Valley district.

Last October, Ms. Lowey, facing a challenge from a progressive upstart, Mondaire Jones, announced her intention to retire after more than three decades in office. She has yet to make an endorsement in the race to succeed her.

At this point, the primary contest appears to be a tossup among four candidates: Mr. Schleifer, who, as an assistant U.S. attorney in California, helped prosecute the “Varsity Blues” case involving the actress Felicity Huffman; Mr. Jones, a former lawyer for the Westchester County Law Department who attended Harvard Law School after growing up poor in Rockland County; Evelyn Farkas, a former Obama administration deputy assistant secretary of defense who handled Russian policy and contends that the Kremlin is working against her; and David Carlucci, a state senator whose district includes parts of Rockland and Westchester Counties.

Mr. Carlucci’s legislative background may actually hurt him: He is a former member of the now-disbanded Independent Democratic Conference, which had collaborated with Republicans in the State Legislature. The American Prospect, a progressive magazine, called him “the Republican-in-Democrat’s-Clothing.”

The recent protests against police violence are expected to help Mr. Jones, the best-known African-American candidate in the race. He has become the chosen candidate of the institutional left, winning the support of Senators Bernie Sanders and Elizabeth Warren, Representative Alexandria Ocasio-Cortez and the New York Working Families Party.

A poll of 1,141 likely Democratic primary voters released on Tuesday found Mr. Jones leading the race with 25 percent of the vote, followed by Ms. Farkas and Mr. Schleifer tied at 14 percent each and Mr. Carlucci at 11 percent. Twenty-four percent of voters were undecided.

“It’s definitely in flux,” said Jim Williams, the Public Policy Polling analyst who conducted the survey, which he said was funded by local Democrats who oppose Mr. Carlucci’s bid.

The candidates are generally in agreement on the issues, including the creation of a robust public health insurance option, although only Mr. Jones supports the creation of a single-payer health care system. They all back the Green New Deal and repealing the federal cap on residents’ ability to deduct state and local taxes from their federal taxes. Several are campaigning on the promise of improving mass transportation options for Rockland County residents, whose access to Manhattan by rail is limited.

With not much separating the candidates on the issues, they have sought to distinguish themselves through their credentials, as well as through personal attacks.

Mr. Carlucci has characterized Ms. Farkas, Mr. Jones and Mr. Schleifer, all of whom returned full-time to the district relatively recently, as carpetbaggers. Mr. Jones, in turn, has labeled Ms. Farkas, Mr. Schleifer and Mr. Carlucci as being more akin to Republicans than Democrats. Other candidates routinely note that Ms. Farkas and Mr. Jones benefit from independent expenditures and donors from outside the district.

Mr. Schleifer’s bountiful spending has prompted comparisons to the former New York City mayor Michael R. Bloomberg’s failed presidential bid, and prompted one of the seven candidates, Asha Castleberry-Hernandez, to lament the laxity of campaign finance regulations that allow for a deluge of personal wealth and outside money.

“It doesn’t have to be that way, especially for people who come from the working class who want to run,” said Ms. Castleberry-Hernandez, a major in the U.S. Army Reserve who now teaches at Baruch College and is the other African-American in the contest. She has raised only $74,000 to run for the seat.

“It’s just an undemocratic process,” she said.

Mr. Schleifer said in a statement that his largely self-funded campaign meant that he did not “rely on those outside interests,” allowing him the freedom to execute his “vision for a more cohesive, creative, and responsive government, guided by science, data, and a deep commitment to justice.”

Nonetheless, his liberal use of his wealth sparked a noteworthy contretemps during a debate hosted on Tuesday by the Business Council of Westchester.

After Ms. Farkas expressed concern “about the big money sloshing around in this election,” and Mr. Schleifer’s unwillingness to divest from pharmaceutical stocks, Mr. Schleifer said she was acting like “a snake.”

“All you know is the fog of the beltway,” Mr. Schleifer said, referring to Ms. Farkas’ decades-long career in Washington. “You talk about being a staffer. You’ve been in the back rooms of Congress for so long that you can’t see straight.”

The district encompasses Indian Point, the nuclear plant that helps power the downstate region and is in the process of shutting down. Not only will the district lose jobs, but questions remain about the safety of the decommissioning process.

While the district is known for its affluence, it contains significant pockets of poverty.

“Westchester and Rockland have two very different DNAs to them,” said Allison Fine, a Westchester-based candidate who counts herself lucky to have a campaign manager from Rockland County well-versed in its intricacies.

“Rockland is not as wealthy as Westchester,” she said. “It has the largest community of Hasidic Jews, second only to Williamsburg. There’s a lot of tension between the religious and secular communities there and they have very little economic growth, which is a huge problem. And then they have transportation problems. Can’t get into the city.”

Some consider the Hasidic vote, which often is delivered in a bloc, as pivotal, and it’s not clear yet which candidate will receive it.

“I think the two candidates that are putting in a lot of effort and getting support from voters in the Jewish community in Rockland are Buchwald, Schleifer,” said Yossi Gestetner, a Town of Ramapo resident and the co-founder of the Orthodox Jewish Public Affairs Council, referring to Mr. Schleifer and David Buchwald, the assemblyman from Westchester.

During a recent visit to New Square and Kaser, towns with heavy Hasidic populations, several residents said they had received phone calls and mailers from Mr. Schleifer’s campaign, but they had yet to develop much of an opinion about him.

Most of the contenders for this congressional seat are making their first run for elective office. Before he served as an assistant U.S. attorney, Mr. Schleifer worked as a former lawyer in Gov. Andrew M. Cuomo’s Department of Financial Services and in private practice. Ms. Farkas spent several years at the Department of Defense and as an employee of the Senate’s Armed Services Committee.

Before serving as a county lawyer in Westchester, Mr. Jones had been in private practice and spent a year working as a fellow at the Department of Justice.

Ms. Fine, a former chairwoman of the NARAL Pro-Choice America Foundation, is the other candidate with experience in public office: She spent two years as an elected trustee in Sleepy Hollow in the 1980s.

Mr. Carlucci and Mr. Buchwald, in contrast, are political veterans.

Mr. Buchwald, a former tax lawyer and physics major, has substantial institutional support from Westchester Democrats like George Latimer, the county executive. Mr. Buchwald helped write state legislation to give Congress access to President Donald Trump’s state tax returns.

“All those things come together to create a solid skill set” that would serve the residents of the lower Hudson Valley well, Mr. Buchwald said.

In an interview, Mr. Jones said Mr. Buchwald’s institutional support would mean little to voters.

“People want to be inspired,” Mr. Jones said. “This is the United States Congress.”

Nate Schweber contributed reporting.