Powerful political adviser David Garth, who spearheaded the creation of the modern political TV commercial and helped elect governors, senators and mayors, has died at age 84.

He died Monday at his home in Manhattan after a long illness, said longtime colleague George Arzt, who served as former Mayor Ed Koch's press secretary.

Garth never held office but was instrumental in shaping New York's government and political process. The mayors he helped elect -- John Lindsay, Koch, Rudy Giuliani and Michael Bloomberg -- served 38 of the last 45 years, and he helped Gov. Hugh Carey capture office in 1974.

Artz said no one in his lifetime ever talked about Garth "without putting genius in front of his name."

"He's probably had more of an impact on the history of the city since the 1960s than any one person," Artz said. "There probably will never be anyone like him again."

Garth, who worked for Democrats and some moderate Republicans, was known for being fiercely competitive with a high success rate on Election Day. He was an outsized figure who was known for his salty language and became the basis for the political mastermind in the Robert Redford 1972 movie "The Candidate."

He insisted that his candidates speak directly to voters, often in direct-to-camera commercials, and admit when they made mistakes. He ran New York's first television-based campaign in 1965, helping little known Rep. John Lindsay capture City Hall.

Garth helped another underdog, Koch, defeat Mario Cuomo for mayor in 1977 and so impressed the vanquished candidate that he asked Garth to help his own gubernatorial campaign years later.

Garth also brokered Giuliani's late 2001 endorsement of Bloomberg, a political neophyte who was trailing in the polls until he received the blessing of Giuliani, the outgoing incumbent hailed as a national hero for his leadership after the Sept. 11, 2001, terror attacks.

"Each time, he took on a candidate he believed in at a time when New York was in crisis," Bloomberg said in a statement. "His success in political campaigns can be seen in his extraordinary won-loss record, but his greatest achievement is the sum of his work: a city and state that are stronger than ever."

Among Garth's other clients: Governors Ella Grasso of Connecticut and Brendan Byrne of New Jersey, Senators Arlen Specter and John Heinz of Pennsylvania and Mayor Tom Bradley of Los Angeles. He also worked with Adlai Stevenson during his unsuccessful 1960 presidential campaign.

Garth's funeral will be private, with a memorial service to be scheduled later.