James H. (Jack) Pollack, one of the last big city political bosses and a power in Maryland politics for a half-century, died yesterday in Baltimore after a long bout with cancer. He was 78.

Mr. Pollack, an ex-prizefighter, built a neighborhood political machine so powerful that he dominated Baltimore politics -- and often the state legislature -- as the pre-eminent boss in the state for decades.

He was a master of the old-time, smoke-filled backroom school of politics, a man who enjoyed being called a political boss and playing the part of the political rogue. His brand of politics was a rough and tumble one; friends were rewarded with patronage plums; enemies were cut off.

"There was a time he had the whole city delegation to the legislature in his pocket," one longtime Baltimore political observer said yesterday. "You couldn't get elected in the city unless you were on a Pollack ticket."

A host of city and state political figures got their start in Mr. Pollack's Northwest Baltimore Organization. The most notable was Gov. Marvin Mandel, who split with his old mentor in 1962, but said yesterday his passing "marks the end of an era."

Mr. Pollack never held an elective office, and only once an appointive one -- a minor office at that, as a member of the State Athletic Commission.

His most remarkable characteristic was his durability. "People have been writing his political obituary for decades, but everytime they wrote it, he'd come back with a new bunch of men and get them elected," said Baltimore City Council President Walter Orlinsky. "He certainly was no longer the boss of the bosses, but he held considerable power right up to the end."

His organization lost its citywide dominance after he bucked Gov. J. Millard Tawes' successful election bid in 1962, but he remained an important factor in close elections right up until his death.

In 1974, for example, Mr. Pollack and his political power and helped elect a political unknown, William A. Swisher, as city prosecutor. Last spring, he was one of the first of the city bosses to jump aboard the bandwagon of California Gov. Jerry Brown Jr. and help deal Jimmy Carter a startling setback in the Maryland presidential primary.

Mr. Pollack, born of Polish-Jewish immigrant stock on Baltimore's east side, was orphaned at age 15. A tough street fighter, he quickly found his talents in demand: first in the professional ring where he fought as a light heavyweight, and later as a street enforcer in the rough Prohibition era.

At one point he was indicted for the murder of a watchman in a liquor-hijacking incident. The case, however, never came to trial, and Mr. Pollack's arrest records mysteriously disappeared from city court records.

He jumped into politics in 1926, and was hired by William (Uncle Willy) Curran, then Baltimore's reigning political boss, to tear down posters put up by Curran opponents. When he moved two years later to the rapidly growing northwest area of the city where many Jewish residents had settled, he became Curran's district leader.

Mr. Pollack soon became an independent power on his own, and Baltimore's first Jewish political boss. His machine was based in the Trenton Democratic Club, which he formed, and the favors he did for residents of the Northwest.

He operated supreme in the area for almost four decades, building up a personal fortune in real estate speculation and the insurance business. His power reached its height during the 1940s and 1950s, when he helped elect his boyhood friend, Thomas D'Alesandro Jr., mayor three times, and would stand in the hallways of Annapolis literally controlling the General Assembly through his handpicked henchmen.

Mr. Pollack, however, lost his last political fight earlier this year -- an attempt to have his grandson, James Dorf, 22, appointed to a vacant seat in the Maryland House of Delegates caused by the death of Del. Murray Abramson.

Dorf was originally named to the seat by the Northwest district's Democratic Central Committee, with his sister, jayme Dorf, casting the deciding ballot. But a judge threw out the selection, declaring Miss Dorf could not vote on the board because she wasn't a resident of the city.

Mr. Pollack is survived by his wife, Carita, a son, Morton C., a daughter, Mrs. Paul Dorf, a stepdaughter, Edwina Blondheim, and six grandchildren.