Former senator Jacob K. Javits, 81, a New York Republican who defined and personified the liberal wing of his party during the 24 years he served in the Senate, died last night in a hospital in West Palm Beach, Fla. He had a form of amytrophic lateral sclerosis, also known as Lou Gehrig's disease.

Javits died at Good Samaritan Hospital after being brought in earlier yesterday with breathing problems, a nursing supervisor said.

In the Senate, Javits was an authority on foreign affairs, a supporter of social welfare measures and an important leader in the fight for the landmark civil rights legislation passed between 1957 and l964.

After he left public office, though he was burdened by an illness that had left his body a husk, confined him to a wheelchair and forced him to take air through a portable respirator, his mind remained clear. With his remaining energy, he struggled to stay active.

"I was an active man, strong and in close touch with the world," he told a reporter in 1984. After contracting his illness, he said, "I had to think about what to do . . . and I decided I had to keep on as long as I could."

He spoke at college commencements, testified before congressional committees and reviewed portions of the papers he compiled and produced over his long public career.

In a statement last night, President Reagan praised Javits' "remarkable courage" in his battle against the crippling disease.

"Throughout his many years in the Senate, Jacob Javits was known for his intellect, for his integrity, for his dedication to the people of New York and the nation and for the sheer joy he took in every day of his work," Reagan said. "Jacob Javits remained to the end a man in love with life."

Javits came to Capitol Hill in 1947 as a member of the House of Representatives. From 1955 to 1957, he was attorney general of New York. He then was elected to the Senate. When he left in 1981, he had served in that body longer than any other New Yorker. He was the ranking Republican on the Foreign Relations and the Labor and Human Resources committees and was a senior member of the Joint Economic and Government Operations committees.

His authority transcended his assignments. He liked to refer to himself as "a Senate man," a term that connoted not only a special respect for the powers and privileges of that institution, but also a determination to occupy a special place in it. He had the qualities -- loyalty, a sense of fairness, and a capacity for hard work, among others -- to be a man of the Senate, and his colleagues accorded him a special standing.

So did their staffs. In a poll of Capitol Hill aides conducted by Congress Watch, a Ralph Nader organization, this Jewish, liberal Republican -- a rare combination in Senate terms -- was voted the brightest and second most influential member.

He was a staunch supporter of New York state, New York City and the state of Israel. Beyond that, some of his most telling work was done as a member of the Committee on Labor and Human Resources. This included his role in creating the Legal Services Corporation and his support for legislation such as the Comprehensive Employment and Training Act and funding for projects for the handicapped and for the arts.

A cause for which he fought throughout his public life was a national health insurance plan.

In his 1981 autobiography, "Javits: The Life of a Public Man," he wrote:

"The achievement that gave me the greatest satisfaction in the labor field is the Pension Reform Act of 1974." The act regulates private pension plans for an estimated 30 million workers and requires that they be funded and viable. Javits drafted the law and fought for it for seven years before it passed.

His interest in foreign policy was longer than his career in Congress. In the House, he was a member of the Foreign Affairs Committee and in 1969 he won a long-coveted seat on the Senate Foreign Relations Committee. He became one its most articulate and best known members and he used it as a forum for his growing opposition to the war in Southeast Asia.

In his memoirs, he said he initially favored the effort in Vietnam and that he did not change his position until 1967. Two years later, he and his wife Marian were tear-gassed while watching an antiwar demonstration in Washington. By 1970, he was addressing antiwar rallies.

He worked for the successful passage of what became known as the Cooper-Church Amendment (to a foreign military sales bill), which barred funds for military operations in Cambodia. He also supported the McGovern-Hatfield Amendment (to a defense procurement act), which sought to cut off all funds for combat activities in Vietnam. It was defeated.

Javits saw a larger question than the war itself: the increasing use of war powers by the president without consulting Congress. The result was the War Powers Act. Opposed by President Nixon, it passed into law over his veto on Nov. 7, 1973.

A measure of Javits' abilities in the Senate was the broad support he mustered for the act, which asserted the constitutional principle that the power to declare war lies with Congress. The law set limits on how long a president can conduct hostilities without congressional consent.

In New York, Javits drew from as many constituencies as he did in the Senate. In 1954, he was the only Republican to win statewide office when he upset Franklin D. Roosevelt Jr. for state attorney general. Two years later, he defeated New York City Mayor Robert F. Wagner Jr. for the Senate. Running for reelection in 1962, he beat his Democratic opponent by nearly a million votes and ran more than 400,000 votes ahead of Republican Gov. Nelson Rockefeller, who also was reelected.

In 1968 and 1974, he took just under half the ballots to win three-way races against Democratic and conservative opposition. His support in the GOP remained strong, however, and it was not until 1980 that he had to run in a Republican primary.

He lost to Alfonse D'Amato, now New York's junior senator. In an effort to make a comeback, Javits ran on the Liberal ticket in the general election. But his day was done. It ended just as his party took control of the Senate, and in his autobiography he wrote:

"After 24 years in the Senate minority, having become the ranking minority member on the Foreign Relations Committee, I was dismissed in the year that my party became the majority in the Senate, when I would have become chairman of the committee -- a lifelong dream. I was shown the promised land but was not permitted to enter it."

Of his contributions in the domestic field he wrote: "I had a hand in shaping almost all of the labor legislation that has come before the Senate, including fair minimum standards for workmen's compensation, occupational safety and health, child labor laws, minimum wage, and the expansion of job training and job opportunities for youths and minorities."

In retirement, his body devastated by disease, it took Javits nearly three hours just to prepare to spend his day. He was unable to lift a hand to sign his name. He wore a stiff collar to keep his head from slumping onto his chest. A visitor found him taking soup through a straw.

But he continued to think and make his views known on politics and public affairs. He organized his library at the State University of New York at Stony Brook. He spoke out in favor of a nuclear freeze and a nonmilitary solution to the problems of Central America. He supported Ronald Reagan in 1980 and again in 1984.

Recalling the advice of a boxing instructor of his youth to "hit 'em from where you are," he said "that's what I'm doing.

"I'm sick. Maybe I won't live much longer, but I'm hitting 'em from where I am. I'm not giving up a thing."

Speaking of his illness to a group of physicians, he said it is important for a terminal patient "to keep the brain in order and functioning. This is the essence of life."

"You must remember, my own philosophy is that you don't belong only to yourself," he said. "You have an obligation to the society which protected you when you were brought into the world, which taught you, which supported you and nurtured you. You have an obligation to repay it."

Jacob Koppel Javits was born on May 18, 1904, on Manhattan's Lower East Side, an enclave that was isolated from the rest of the country by poverty, religion, language and social customs.

His father Morris Javits had been a Talmudic scholar in his native Ukraine and became a tenement janitor and Tammany Hall worker in New York. The future senator's mother, the former Ida Littman, came to the United States from what was then Palestine and was illiterate at the time of her marriage.

As a youngster, Javits helped his mother sell clothes from a pushcart. He wrote that "working a pushcart" taught him "how to attract attention and make a point. I have never felt the least embarrassment or shyness about getting up in front of a crowd -- even a hostile political one -- to speak, argue or to explain and persuade."

Those early experiences went even deeper: "My personal commitment is rooted in my boyhood on the Lower East Side of Manhattan, and it has lived with me all these years; that commitment and my belief in a world rule of law to replace the rule of force were my reasons for being in public life."

He disliked his father's work for the Democratic Tammany machine. In elections, the elder Javits would circulate through the tenements paying residents to vote the "right" ticket. "I felt a revulsion against this corruption," he recalled, "and it was one of the main reasons I joined the Republican Party rather than the Democrats when I grew up."

After graduating from high school, Javits became a traveling salesman of lithograhic supplies and the operator of a debt collection business. He attended Columbia University and earned a law degree at New York University.

He and his brother Ben established a law practice together. Javits was a trial lawyer, often representing small stockholders against large corporations. He was "blackballed" the first time he applied for membership in the Bar Association of New York.

His fascination with politics led him to begin working in city elections in the 1930s. In World War II, he served in the Army, reaching the rank of lieutenant colonel and earning the Legion of Merit. In 1946, he was elected to the U.S. House of Representatives.

While his legal and political career progessed smoothly, his personal life was sometimes difficult. In September 1933 he married Marjorie Joan Ringling, a Catholic and the daughter of circus owner Alfred T. Ringling.

Javits wrote that "my marriage to Marjorie Ringling was a recreation marriage, a good-times marriage -- and that was not enough. When the recreation gave out the marriage gave out." They were divorced in 1936.

In 1947, in his first year in the House, he married Marian Ann Borris. Javits wrote, "Early in my relationship with Marian, her mother had been concerned when I asked my secretary to telephone a message arranging a date instead of placing the call myself; she felt that was not the proper way to do things if romantic feelings were involved."

This part of the courtship foreshadowed some aspects of the marriage. On the wedding day, Javits cut the festivities short so he and his bride could take the overnight train to Washington. He wanted to attend a special session of Congress.

He wrote, "It must have been eleven o'clock or later when we boarded the train, Marian bedecked in pearls, in a form-fitting gray sweater set, I carrying my usual stack of morning newspapers. From the very beginning, work and world affairs were on a collision course with marriage."

"Increasingly, my world became one of politics and government, and she turned to the artistic and intellectual side of life, which she felt she could find best in New York. Whether our physical separation was the result or cause of this developing pattern of our marriage is difficult to say; it was probably something of both. Slowly, with enduring mutual respect and loyalty, but not without some pain on both sides, we began to work out the modus vivendi of separate interests -- two lives interacting and intersecting but not congruent -- that characterizes our marriage to this day."

The marriage survived. But it is true, perhaps, that the great love of "Jake" Javits' life was the Senate.

"For 24 years the United States Senate was my home," he wrote. "I know its rules and traditions and moods, and I could often sense what action it was about to take. My seat on the aisle in the Senate chamber became as agreeable to me as a favorite living-room easy chair, and I was familiar with the voices, the quirks, the foibles and inclinations of the ninety-nine other senators."

In that seat he always tried to live by his principles. "I never relied particularly on charisma," he said, "nor did I generally carry the banner for one burning emotional issue that could bewitch voters into following their hearts instead of their heads."

Survivors include his wife and three children.