Arthur J. Finkelstein, left, and Donald Curiale, his husband, in 2013. (Gary Maloney/Gary Maloney)

Arthur J. Finkelstein, whose sharp, relentless attack ads helped elect dozens of conservative political candidates in the United States and abroad and made him a kingmaker in Republican circles for decades, died Aug. 18 at his home in Ipswich, Mass. He was 72.

The cause was metastasized lung cancer, his family said in a statement.

Mr. Finkelstein cultivated an image as a reclusive, behind-the-scenes figure, seldom granting interviews and rarely drawing attention to himself in public — all of which lent him a mystique as a pollster, campaign manager and ruthless operative in electoral politics.

He became a prominent political power broker in the 1970s who helped propel the careers of Republican senators such as James L. Buckley (N.Y.), Jesse Helms (N.C.), Orrin G. Hatch (Utah) and Alfonse D’Amato (N.Y.), as well as Israeli Prime Minister Benjamin Netanyahu. Mr. Finkelstein’s influence extended to two generations of Republican political consultants who launched their careers on his campaigns.

Mr. Finkelstein was considered a master at developing simple campaign messages, which were repeated in such a steady barrage of negative television commercials that he was sometimes called the “merchant of venom.” As much as anyone, he was responsible for making the word “liberal” a political slur.

Arthur Finkelstein, right, with Sen. Alfonse D'Amato (R-N.Y.) in 1998. (Gary Maloney)

He was also something of a political conundrum — especially after it was revealed in 1996 that his private life as a gay man was in sharp contrast to the views of some of the conservative firebrands he helped elect. Helms, for instance, often railed against the “homosexual movement,” which he said “threatens the strength and the survival of the American family.”

In 1996, New York Times columnist Frank Rich described Mr. Finkelstein as someone who “sells his talents to lawmakers who would outlaw his family’s very existence.”

Mr. Finkelstein was credited with helping raise Ronald Reagan’s national profile during the 1976 Republican primary campaign. Ultimately, the nomination went to President Gerald R. Ford, who lost the general election to Democrat Jimmy Carter.

Reagan’s insurgent campaign against a sitting president laid the groundwork for his overwhelming presidential victory in 1980. Mr. Finkelstein was seen as one of several GOP strategists, including Roger Ailes, Lee Atwater and Charlie Black, who were instrumental in helping shape what became known as the Reagan Revolution.

“Without Arthur Finkelstein, Ronald Reagan might never have become president of the United States,” historian and Reagan biographer Craig Shirley wrote on the website of National Review magazine in January 2017.

During Reagan’s eight years in the White House, Mr. Finkelstein was an informal adviser to the administration and managed congressional and gubernatorial campaigns across the country.

“He uses a sledgehammer in every race,” political scientist Darrell M. West told the Boston Globe in 1996. “I’ve detected five phrases he uses — ultraliberal, superliberal, embarrassingly liberal, foolishly liberal and unbelievably liberal.”

Mr. Finkelstein was 25 when he helped Buckley, a registered member of New York’s Conservative Party, win a six-person race for the Senate in 1970. Two years later, Mr. Finkelstein’s polling and political guidance helped Helms become the first Republican elected to the Senate from North Carolina since the 19th century.

As other victories piled up, Mr. Finkelstein became one of the GOP’s top campaign masterminds and the chief strategist of the National Conservative Political Action Committee. In 1980, his efforts helped defeat such longtime Democratic stalwarts as Sens. George S. McGovern (S.D.) and Birch Bayh (Ind.).

Mr. Finkelstein was also the architect of D’Amato’s unexpected 1980 Senate victory in New York, where he upset four-term incumbent Jacob K. Javits in the Republican primary.

Following Mr. Finkelstein’s recommendation, D’Amato — who once trailed in the polls by more than 60 points — made Javits’s little-known diagnosis of Lou Gehrig’s disease an issue in the campaign. One often-aired commercial, reportedly written by Mr. Finkelstein, ended with the line, “And now, at age 76 and in failing health, Jacob Javits wants six more years.”

D’Amato won a three-way general election against Democratic Rep. Elizabeth Holtzman and Javits, who entered the race as a third-party candidate after his loss in the primary. D’Amato, who went on to serve three terms in the Senate, once said that Mr. Finkelstein’s “political skills are second to none.”

In 1994, Mr. Finkelstein was the driving force behind Republican George E. Pataki’s upset victory over New York Gov. Mario Cuomo and devised the campaign’s inescapable slogan: “Too liberal for too long.”

Dozens of well-known Republican political operatives worked on campaigns with Mr. Finkelstein over the years, including Ari Fleischer, Roger Stone, Alex Castellanos, Frank Luntz, Beth Myers, Tony Fabrizio, Gary Maloney and Larry Weitzner.

In National Review, Shirley, the Reagan biographer, called Mr. Finkelstein “simply the greatest and most controversial, the most ethical and most successful political consultant and pollster in the history of American politics.”

Others were not so charitable.

“He makes James Carville look like Mary Poppins,” GOP stategist Mike Murphy told The Washington Post in 1996, referring to Bill Clinton’s 1992 campaign manager. “He has contempt for a lot of the party establishment. He is brilliant, crazy and secluded.”

A 1996 CNN report described Mr. Finkelstein as “the stuff of Hollywood: A man who can topple even the most powerful foes, yet so secretive that few have ever seen him.”

He registered at hotels under assumed names and avoided photographers.

While directing Netanyahu’s successful campaign for Israeli prime minister in 1996, he described his approach to politics in a rare interview with the New York Times magazine.

“I think I’m the playwright or the director, and not the actor,” he said. “And the actors need to be onstage, not the director. And I think it’s absurd that people who do what I do become as important, as celebrated, as the ones who are running.”

Arthur Jay Finkelstein was born May 18, 1945, in Brooklyn and grew up in Queens and later Levittown, N.Y. His father was a cabdriver.

Mr. Finkelstein adopted hard-line conservative views early in life and, in his teens, supported the 1964 presidential bid of conservative Sen. Barry Goldwater (R-Ariz.).

While attending Queens College, a branch of the City University of New York, he worked on a radio program with author Ayn Rand, whose writings have had a strong influence on generations of conservative and libertarian thinkers.

After graduating in 1967, Mr. Finkelstein worked in the computer department of NBC News. When he began his career in politics, he used polling results to build campaign themes around voters’ social, financial and emotional needs.

Mr. Finkelstein’s office in Irvington, N.Y., was managed by his brother Ronald Finkelstein, but he lived for many years in Ipswich and Fort Lauderdale, Fla.

In 1996, Boston magazine noted that Mr. Finkelstein was gay and lived with a longtime partner, Donald Curiale, and their two daughters. When Mr. Finkelstein and Curiale were married in 2005, few if any of his political clients attended the wedding.

Survivors include his husband, of Ipswich and Fort Lauderdale; two daughters, Jennifer Delgado of Danvers, Mass., Molly Baird-Kelly of Alpharetta, Ga.; two brothers; and a granddaughter.

After engineering Netanyahu’s victory in Israel in 1996, Mr. Finkelstein increasingly concentrated on international elections. He helped Ariel Sharon become Israel’s prime minister in 2001 and was active in electoral campaigns in Albania, Austria, Bulgaria, Hungary, Kosovo, Ukraine and other countries.

Mr. Finkelstein said he considered himself a libertarian, and the candidates he supported in the United States were exclusively Republicans.

“The political center has disappeared,” he told the Israeli newspaper Maariv in 2004, “and the Republican Party has become the party of the Christian right more so than in any other period in modern history.”

As the party swung further to the right, Mr. Finkelstein did not acknowledge any role he may have played in that shift. But in another respect, his political instincts proved prophetic, more than a decade before the 2016 presidential election.

“In terms of the Republicans,” he quipped to Maariv in 2004, “Hillary Clinton is a wonderful candidate for the presidency.”