William Loeb, 75, the crotchety, conservative newspaper publisher whose gift for invective and base in New Hampshire gave him vastnotoriety in American journalism and a unique influence in national politics, died yesterday of cancer.

As the vituperative and idiosyncratic proprietor of the Manchester Union Leader, the only statewide paper in the state that holds the nation's first presidential primary, Mr. Loeb found himself every four years with a national forum for his scathingly denunciatory brand of personal journalism.

Known over the years for applying such epithets as "Dopey Dwight" to Dwight Eisenhower and "Jerry the Jerk" to Gerald Ford, Mr. Loeb is considered to have played a key role in the collapse of Sen. Edmund Muskie's 1972 presidential bid.

The Maine Democrat's campaign for his party's nomination appeared to fizzle after an emotional display in which he stood in the snow outside the Union Leader building to condemn Mr. Loeb for the publication of an article on Muskie's wife that the senator found offensive.

A man to whom dullness and blandness were journalistic sins, Mr. Loeb, who at one time or another called former Sen. Eugene McCarthy a "skunk," Richard Nixon "Tricky Dicky" and a "bungler," and who described Jimmy Carter as an "out and out leftist coated over and disguised with peanut oil," appeared unruffled by frequent outcries against what critics called his excesses and petty personal attacks.

Often impressing interviewers as polite, genteel and mild-mannered in person, Mr. Loeb responded to criticism by asking: "If they can't take it from me, what will they be able to take in times of a real crisis?"

In a statement, President Reagan said he and his wife were saddened by Mr. Loeb's death, and called him "a patriot and a man of deep conviction who had the courage to speak frankly and forcefully."

Self-described as a philosophical rather than financial conservative, Mr. Loeb, who lived the life of a country squire in a home in Pride's Crossing, Mass., sometimes likened himself to Theodore Roosevelt, for whom Mr. Loeb's father had worked.

The elder Loeb was Roosevelt's private secretary when William Loeb 3d was born in Washington on Dec. 26, 1905, and the president was the future publisher's godfather.

Mr. Loeb studied at the Hotchkiss School in Connecticut where, he said, he was "sort of an 'aginner' who didn't always fit in," and at Williams College where he earned honors in philosophy. At Williams, Loeb -- a Baptist -- quit a fraternity to protest its refusal to admit a Jew.

While still in college he began part-time newspaper work and after two years at Harvard Law School went into journalism full-time as a Hearst news service reporter. He subsequently held several reporting jobs, and worked between them in sales and public relations.

In an incident that seemed to prefigure the sometimes quixotic brand of personal journalism he later practiced, Mr. Loeb as a novice journalist once became so outraged at seeming unfairness in the conduct of a panel discussion he was covering that he plunged into the discussion himself.

When Japan invaded China in 1937, Mr. Loeb joined a group calling for the boycott of Japanese goods. After it was revealed that the group was Marxist, Mr. Loeb was described as angry, embarrassed and determined from then on to combat communist influence.

"Seen from the viewpoint of that age, I was an idealist," he said, "but seen from the viewpoint of today, I'm called reactionary. I just haven't changed much from those days."

In 1941, with his mother's financial aid, he bought the St. Albans (Vermont) Daily Messenger, and the next year acquired the Burlington (Mass.) Daily News. Buying into the Union Leader in 1946, he gained complete control of it two years later. In 1948 he also acquired the New Hampshire Sunday News, which became the Sunday edition of the Union Leader.

Arriving in Manchester to find it a deteriorating town whose textile mills were declining, Mr. Loeb was credited with making his newspaper an aggressive civic watchdog, exposing conflicts of interest and government waste and corruption.

Iconoclastic, not always predictable, Mr. Loeb was a vigilant anticommunist who vigorously supported the labor movement and backed Wisconsin Sen. Joseph R. McCarthy, but once called Robert Welch, head of the archconservative John Birch Society, a "bloody nut."

Mr. Loeb, who in his writing was a stranger to subtlety, expressed his views bluntly and unequivocally in editorials that ran on the front page of his paper, often printed in three colors, in a melange of formats and typefaces that were as quirky and idiosyncratic as the man himself.

Acknowledging that he sometimes overstated the case in his editorials, he asserted that he did so to "stir the pot" of thought and discussion about public issues. Similarly, he defended his treatment of political candidates by asserting the belief that the voters had the right to see each of them "at his worst as well as at his cellophane-wrapped, pre-packaged best."

He also admitted that he was outrageous in part because it sold newspapers. When Mr. Loeb took over the Union Leader, circulation stood at about 40,000. By 1972 it was up to 63,000. What was his secret? "Don't be dull," he said.

In the 1980 campaign, a Union Leader editorial told Democratic primary voters that their choice was between the "stupid, the coward and the flake," referring respectively to President Carter, Sen. Edward M. Kennedy of Massachusetts, and California's Gov. Jerry Brown.

On the Republican side, Mr. Loeb supported Ronald Reagan, and his paper portrayed George Bush as soft on the Soviet Union and printed an "expose" on another contender, Rep. Philip Crane of Illinois, that depicted him as a philanderer and drinker.

Although the extent of Mr. Loeb's influence on presidential politics is difficult to assess, he apparently had greatest impact in 1972, when Muskie approached the New Hampshire primary as the front-runner for the nomination.

Shortly before the balloting, Mr. Loeb printed a letter in which Muskie was accused of referring to French Canadians as "Canucks," an ethnic slur. The letter later was alleged to be a product of a Nixon campaign "dirty tricks" operation.

Mr. Loeb charged that Muskie found the slur "amusing," and followed with an editorial quoting Mrs. Muskie as saying on a campaign trip, "Let's tell dirty jokes."

Muskie responded by standing on a flatbed truck outside the Union Leader office and calling Loeb "a liar" and "a gutless coward."

When Muskie spoke of Loeb's attack on his wife, observers saw tears, and the show of emotion appeared to deflate his campaign. The nomination went to Sen. George McGovern of South Dakota.

"Muskie should have paid absolutely no attention to me," Mr. Loeb said. "But no -- they made it seem like he was Eliza crossing the ice."

In August 1979, Mr. Loeb said he planned to turn over ownership of 75 percent of the newspaper to a trust for his employes. Shortly before, he had agreed to settle a suit by selling 25 percent of the newspaper's stock.

Mr. Loeb said that he had planned for 30 years to turn the paper over to the employes.

"After I die, I want the paper to be run by people who share the same philosophy that I do, that of public service," he said.

At one time he borrowed money for the Teamsters Union, and his friendship with Jimmy Hoffa grew into strong support of the controversial union head.

Mr. Loeb was married three times. He and his second wife had one daughter. His third wife was Nacky Scripps Loeb, and they had two daughters.

Mr. Loeb died at the Leahy Clinic in Burlington, Mass, according to an official of the Loeb newspapers.