Keith Haller, a prominent Maryland pollster and political strategist who used his encyclopedic knowledge of state politics to advance Democratic candidates and causes such as the construction of the Purple Line light-rail system, died Dec. 11 at his home in Rockville, Md. He was 70.

The cause was cancer, said his wife, Stacy Pagos Haller.

Mr. Haller was president and chief executive of Potomac Inc., a Bethesda, Md.-based survey firm he founded in 1982. Maryland politicians recalled him as being on the cutting edge of survey research, blending sophisticated computer analysis of voter behavior and glossy direct-mailing with the perfection of shoe-leather techniques such as door-to-door campaigning.

He mined precinct-by-precinct voter data to help shape campaigns and inform candidates on public perception of issues and contenders. Sen. Chris Van Hollen (D-Md.) said Mr. Haller “was an encyclopedia when it comes to Montgomery County politics.”

According to a Washington Post profile of Mr. Haller, novice congressional candidate Steny H. Hoyer hired the firm to help him win a 1981 special election for a district that included parts of Southern Maryland as well as Prince George’s and Anne Arundel counties.

Hoyer, a former Maryland Senate president, relied on data from Mr. Haller’s firm showing a likely high turnout among black voters to focus his efforts on winning their support. In a contest that centered on the Reagan administration’s proposed budget cuts, Hoyer prevailed with 55 percent of the vote over a well-funded GOP challenger, Bowie Mayor Audrey Scott, and began his ascent to House majority leader.

Former Democratic congressman Michael Barnes, who represented a Montgomery County district, called him “a brilliant pollster.”

“Nobody knew Maryland or the Washington region better than Keith,” Barnes said. “He had an ability to read the nuances in polls in a way that I found singular.”

Lawyer Steven VanGrack became mayor of Rockville in 1985 with what he once described to The Post as Mr. Haller’s take-charge manner. In a difficult three-way race, VanGrack proposed jogging along congested Rockville Pike during rush hour to draw attention to what he saw as the city’s unlivable traffic. Much of his campaign staff thought it amounted to a “silly” stunt, VanGrack recalled to the newspaper, but Mr. Haller signed off and VanGrack parlayed the extensive TV coverage of his jog into a win.

George Keith Haller was born in West Babylon, N.Y., on Jan. 5, 1948. He received a bachelor’s degree from Utica College in Upstate New York in 1969.

He came to Washington to pursue graduate work at American University but left his studies to participate in the youth movement against the Vietnam War. He was exempt from military service because of a hearing impairment, his wife said.

In addition to his wife, whom he married in 1984, survivors include a son, Michael Haller of Rockville; and a sister.

Mr. Haller was the campaign manager for Barnes’ s 1978 congressional run. The first-time manager shepherded the 35-year-old fledgling candidate — a lawyer and member of the Maryland Public Service Commission — to a surprise victory over the one-term GOP incumbent, Newton I. Steers Jr. He became an aide to Barnes and then worked on Capitol Hill for several years before establishing his own polling firm.

The victory put Mr. Haller in demand for dozens of races over the next several years, including Ida G. Ruben’s successful run for state Senate and O. James Lighthizer’s successful bid for county executive of Anne Arundel County.

Bruce Adams, who said he ran for Montgomery County Council largely at Mr. Haller’s urging, recalled being astonished by Mr. Haller’s grip on voting statistics and how they could be read to predict voter behavior. “He knew all the political statistics about who got what in Precinct 7-4 in 1982,” said Adams, who won his first bid for County Council in 1986. “Who else can remember that?”

Mr. Haller “wasn’t just a hired gun — he was a committed activist,” said Adams, crediting the pollster with coming up with the idea of Montgomery County’s Community Service Day, among other initiatives, more than three decades ago.

“I used to joke with Keith about it — ‘Keith, I get all the credit, but wasn’t this your idea?’ He would never say it was — he would just sort of smile, which told me it probably was.”

Mr. Haller also worked to establish the Purple Line, a 16-mile east-west light-rail connector stretching across Montgomery and Prince George’s counties. He urged Bill Bronrott, who at the time was a state delegate representing Montgomery County, to create a Purple Line legislative caucus in the Maryland General Assembly. He also helped organize students at the University of Maryland around the benefits of the Purple Line, which is slated to have a stop at the college.

Jonathan Sachs, former student government president, said he and Mr. Haller worked to form Terps for the Purple Line. Before Mr. Haller’s involvement, Sachs said, there wasn’t much student activity around the project.

“He just really helped empower us to let our voices be heard and known around the project,” Sachs said.

Gregory Sanders, vice president of the advocacy group Purple Line Now, credited Mr. Haller with helping the grass-roots group make a “big splash” with a tiny budget. Mr. Haller served as a volunteer board member for the group.

The $2.4 billion light-rail line project between Bethesda and New Carrollton, years in the making, survived a legal challenge and political opposition to break ground last year.

“To use a military metaphor, he was always the artillery guy,” Sanders said. “Big impact, figuring out what the long-term looked like, what are the economics, what’s the polling.”

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