Former Virginia governor John N. Dalton, a lawyer who helped build his state's Republican Party into one of the strongest in the South, died here today of adenocarcinoma, a form of lung cancer. He was 55.

Mr. Dalton, a nonsmoker who served as the state's chief executive from 1978 to 1982, was diagnosed in the summer of 1983 as having lung cancer. He then underwent surgery that was thought to have been successful and his name surfaced as a possible candidate for governor in the state's 1985 elections.

His condition worsened, however, and he was hospitalized in December. Since then, he had been receiving treatment at the Medical College of Virginia Hospitals, where he died at 12:51 p.m. He also had been treated at Johns Hopkins Hospital in Baltimore.

Mr. Dalton's wife, the former Edwina Jeanette Panzer, and a sister were with the former governor at his death, said Dr. Wade K. Smith, chief of hematology and oncology at Virginia Commonwealth University.

Smith said Mr. Dalton was hospitalized yesterday after he began experiencing increasing difficulty in breathing.

After his cancer was detected, Smith said, Mr. Dalton noted that he had been "around smoke all my life -- all those smoke-filled rooms" -- and thought that had affected his health.

Mr. Dalton was one of the most popular figures in the Virginia GOP and one of the first to make extensive use of television and computerized mailing lists in his 1977 governor's race against former lieutenant governor Henry E. Howell of Norfolk. Mr. Dalton, who had succeeded Howell in the state's No. 2 job, won the state's highest office by a large margin. He was helped, analysts said, as much by the liberal Howell's own gaffes as by his own shrewd, conservative campaign style.

As governor, Mr. Dalton's administration offered few major initiatives, preferring to continue the efforts begun by his predecessors to bring more industry into the state and run an administration that avoided any major tax increase.

In two controversial moves, he attempted to settle a prolonged dispute with federal officials over desegregation of the state's public colleges and sent state troopers into the coal fields of Southwest Virginia after a violent strike involving miners.

Union officials accused him of strike breaking, and his efforts to end the college dispute earned him the condemnation both of conservatives, who accused him of bowing to the demands of federal bureaucrats, and educators, who said his agreement was unrealistic.

Politically his impact was great. J. Marshall Coleman of McLean, a Republican who served as state attorney general during the Dalton administration, said Mr. Dalton was "part of the transition from the old Byrd machine to modern Virginia politics."

Linwood Holton started the transition as the first Republican to win the governorship. Mills E. Godwin, Mr. Dalton's predecessor, "solidified it, and Dalton put on the final touches," Coleman said.

Throughout his rise, Mr. Dalton was an unprepossessing, some said lackluster, campaigner. He relied heavily on advisers, who found that despite his shyness, Mr. Dalton came across well on television.

Mr. Dalton, who had spent most of his life in Radford, a small town south of Roanoke, carried Northern Virginia by a substantial margin. But he was "probably not as comfortable" in the Washington suburbs as he was in central Virginia or in his native Southwest Virginia, said Paul G. Edwards, a former Washington Post reporter who became one of the governor's press secretaries.

"He was staggered by the transportation needs up there, in terms of cost," Edwards said, but in 1980, faced with the need for more money for highways because of falling gasoline consumption, he got a 2-cent gasoline tax increase through the Democrat-controlled legislature.

He pushed for the tax increase -- Mr. Dalton had wanted twice that much -- even though it went against his goal of trying to operate state government without a major tax increase.

Mr. Dalton "achieved his objective in the governor's office," Mr. Edwards said. "He came in with an accurate sense of what his constituents wanted at that time, and he was ideally suited to deliver it."

After he was succeeded by Democrat Charles S. Robb in 1982, Mr. Dalton became a senior partner in the Richmond law firm of McGuire, Woods & Battle. The Virginia GOP was unable to continue its string of victories, largely because the moderate Robb replaced Howell as the GOP's primary antagonist.

Born July 11, 1931, in the Southside Virginia town of Emporia, John Nichols Dalton grew up in Radford, where his interest in politics was sparked by his uncle, a well-known Republican lawyer named Ted Dalton who adopted him as a teen-ager.

"I can't remember when it wasn't my ambition to serve as governor," the younger Dalton once said, reminiscing about growing up in a home where politics was a staple at the dinner table.

John Dalton and the Republican party matured together in the early 1950s. Ted Dalton, who later became a federal judge, unsuccessfully ran for governor in 1953, as a progressive Republican challenging the entrenched conservative Byrd organization and running on a platform that some would regard as more liberal than the one on which his adopted son would later run.

John Dalton worked in his father's campaign after his graduation from the College of William and Mary, where he made the dean's list and was elected student body president.

After service in the Army, Mr. Dalton earned a law degree at the University of Virginia and returned to Radford, where he went into practice with his father. The younger Dalton was elected state Young Republicans president in 1959, and, capitalizing on the popularity of President Eisenhower, he built the number of YR chapters from seven to 37 during his one-year term.

He was also very successful in business, taking over Ted Dalton's large farming operations and soon adding his own farms and other holdings. By the time he ran for governor at age 45, he was one of the richest men ever to seek public office in Virginia.

Mr. Dalton rejected a chance to run for statewide office in 1961, when the usual Democratic sweep was still assured, but four years later he won a seat in the House of Delegates, after a campaign during which he visited every one of the 8,000 homes in the district.

He was elected to the state Senate in 1972, and in 1973, with GOP fortunes bolstered by switches of former Democrats, he was elected lieutenant governor on a Republican ticket headed by Godwin, the former Democratic governor.

Gov. Gerald L. Baliles, a Democrat who served in the General Assembly during Mr. Dalton's term as governor, said Mr. Dalton "lived a life of courage." Baliles said Mr. Dalton was "proud of his Southwest Virginia heritage, and drew strength from its mountains and its people."

Richard Cullen, a law partner, said "he was here every day -- cancer be damned." Cullen said that Mr. Dalton was at work last Friday, and called in dictation yesterday.

In addition to his wife, Mr. Dalton is survived by four children, Katherine Scott, Ted Ernest, John Nichols Jr. and Mary Helen.