Hal Malchow, a Democratic consultant who merged the rigors of science with the art of politics, using data to help candidates connect more effectively with voters and to change the way modern campaigns are run, died March 21 in Zurich. He was 72.

His death was announced by his family. Mr. Malchow had chosen what he described as physician-assisted death subsequent to progressing Alzheimer’s disease.

Mr. Malchow, who watched his mother succumb to early-onset dementia, underwent genetic testing at 35 and learned that he also was likely to develop Alzheimer’s. He resolved then that if he became ill with the disease, he would end his life before his impairment became too severe.

In an era of politicking when campaign gurus often sold their services on the basis of their supposedly oracular knowledge of the electorate, Mr. Malchow, by all accounts, was a consultant of a different breed.

Political science, he argued, should be just that — a science, like medicine or any other, informed by data gathered in randomized controlled trials, the gold standard of laboratory experimentation.

“If you’re a Republican and Hal Malchow showed up on the other side, you knew it because you were about ready to get hurt,” Karl Rove, the Republican strategist who helped propel George W. Bush to the presidency, said in an interview for this obituary.

Mr. Malchow worked for every Democratic presidential nominee from Michael S. Dukakis in 1988 to John F. Kerry in 2004, more than 30 U.S. senators and 20 governors, and organizations including the Democratic National Committee, the AFL-CIO, the Sierra Club and Emily’s List, which seeks to help elect women who support abortion rights to office.

His expertise lay in what is known as direct voter contact, an area of campaigning that sometimes goes unnoticed in the glare of television advertising but that consumes hundreds of millions of dollars nationally in any given election year.

From the earliest days of his career, direct contact intrigued Mr. Malchow because, unlike TV advertising or campaign rallies, it allows a candidate to reach voters personally. He saw the electorate not as “a puzzle of blocs and zones” but rather as “an array of individuals,” journalist Sasha Issenberg wrote in his 2012 book, “The Victory Lab: The Secret Science of Winning Campaigns.”

Old-school direct voter contact encompasses tools such as mailers (Mr. Malchow’s specialty), robocalls (which, he cautioned his clients, did little — if anything — to bump turnout) and door-to-door canvassing (more expensive but also far more effective, he emphasized, than phone calls).

The key in any outreach, he insisted, was to test different approaches to a given flier or other communication, gather data on the responses among potential voters or donors, and adjust the campaign accordingly.

“In politics, we spend hundreds of millions — even billions — of dollars on a whole arsenal of methods to turn out voters. But we spend almost nothing to find out what works,” he wrote in the publication Campaigns & Elections in 2004. “The absence of real empirical knowledge about these questions by campaign professionals tells a sad and shocking story about how we do business.”

Mr. Malchow conceded that Republicans had pursued such empirical data before Democrats did. But within liberal circles and beyond, the methodology he championed was widely credited with having helped to upend “much of what the political world thought it knew about how voters’ minds work, and dramatically changed the way that campaigns approach, cajole, and manipulate them,” Issenberg wrote.

Mr. Malchow spent years perfecting direct mail to keep it out of junk-mail piles. He rejected glossy color pamphlets, which are lucrative for the consultants who draw them up, in favor of simple missives printed on white paper. He had found the plain mail to be more persuasive to voters.

“Generally, the most important mail has the least design to it,” Mr. Malchow told NPR in 2012, noting that “the IRS never puts any pictures or colors on their envelopes.”

At its core, his insight was that “the more political you are, and the more inauthentic in your messaging, the less effective it is,” said Doug Sosnik, who served under President Bill Clinton as White House political director. “That’s now the coin of the realm in all politics because there is such alienation toward politicians and politics.”

As campaigns moved increasingly into the digital sphere, Mr. Malchow’s consulting firm, MSHC Partners, one of the largest voter-contact outfits in the United States, became “the first traditional [Democratic] firm to successfully offer internet marketing services to clients,” according to the political publication Hotline.

Mr. Malchow was recognized as a progenitor of microtargeting, the use of large data sets to identify voters likely to support a candidate or cause based on characteristics such as their age, race or education — or even consumer data such as ownership of an electric or hybrid vehicle.

He observed that voters generally respond better to communications that offer them new information rather than well-trodden partisan lines.

Mr. Malchow was once retained by a “terribly overmatched challenger to a powerful GOP state legislator in a solidly red district,” recalled Donald P. Green, a Columbia University political scientist who has studied the application of data to politics.

Knowing that the client’s campaign war chest would permit only a single mailing, Mr. Malchow and his colleagues dug for a kernel of information voters might not know. Reviewing the incumbent’s voting record, they found that he had once voted to allow dog hides to be used for industrial purposes.

“A postcard of a cute dog pleading not to be made into a coat became the sole salvo of the challenger’s campaign,” Green wrote in an email. “The challenger lost by a handful of votes. The fact that it was close at all had only one explanation.”

Harold Clark Malchow Jr. was born in Gulfport, Miss., on Nov. 4, 1951. His mother was an economist, and his father was an engineer.

Under the influence of his mother, Mr. Malchow was raised to support Republicans. She was deeply upset, Issenberg reported, when desegregation brought African American students to her son’s ninth-grade class. (His father secretly voted for President Lyndon B. Johnson, a Democrat, in 1964.)

During the civil rights era, Mr. Malchow was drawn into liberal causes. At Millsaps College in Jackson, Miss., where he received a bachelor’s degree in political science in 1973, he helped organize rallies against the Vietnam War and joined the Mississippi Freedom Democratic Party, which was founded to resist segregationists in the state Democratic Party.

One of the first campaigns he worked for was the unsuccessful 1971 gubernatorial bid of Charles Evers, the brother of the slain civil rights activist Medgar Evers.

After receiving a law degree in 1981 from the University of the Pacific in California, Mr. Malchow had his first major win as campaign manager for then-U.S. Rep. Al Gore (D-Tenn.) in the 1984 election when Gore won a U.S. Senate seat.

Mr. Malchow, who collected many of his research findings in the 2003 book “The New Political Targeting,” disbanded his consulting firm in 2010.

“Advertising has become not just more negative but more vicious and personal,” he explained at the time, adding that “I just no longer want to do this work.”

With Michael Podhorzer, a former political director of the AFL-CIO, Mr. Malchow helped found the Analyst Institute, a self-described “clearinghouse for evidence on voter contact and engagement programs” serving liberal candidates and causes. In his spare time, he published several fantasy novels and political thrillers.

Mr. Malchow’s marriage to Astrid Weigert ended in divorce. Survivors include his wife of nine years, Anne Mahoney Marsh; a son from his first marriage, Alex Malchow; two stepsons, Michael Marsh and Timothy Marsh; two brothers; and a sister. Mr. Malchow lived for years in the Washington area before moving to Santa Fe, N.M., in 2018.

Mr. Malchow was not eligible for physician-assisted suicide under U.S. laws, which require the patient to be terminally ill with only months to live. He did qualify in Switzerland, which recognizes a right to euthanasia for patients with “incurable, permanent, severe psychological disorders.” Issenberg wrote about Mr. Malchow’s euthanasia decision last week in Politico.

Earlier this year, even as his Alzheimer’s disease worsened, Mr. Malchow self-published a book, “Reinventing Political Advertising,” urging Democrats to overhaul their approach to mass communications. Noting the diminishing sliver of undecided voters and ticket-splitters, he challenged the left to build support for the Democratic Party as a whole, not just specific candidates, as has been the long-standing practice.

As his death neared, Mr. Malchow joked to Issenberg that he took comfort in the knowledge that, whatever the outcome of the 2024 presidential race, he would never again see former president Donald Trump, the presumptive Republican nominee, in the White House. The result of the election, Mr. Malchow said, lies at least partly in the hands of Democratic strategists.

“They’re doing their targeting in exactly the wrong way,” he told Issenberg, wryly adding that “if I ever go back to Washington … the media consultants will kill me.”