Kennedy, 77, served as a senator from Massachusetts from 1962 until his death. He championed such liberal causes as abortion rights and "gay marriage," but he also led successful legislative efforts in support of religious freedom and tobacco control.
He was the sole surviving son of a famous political family that produced not only a United States president but also a U.S. senator who appeared possibly in line to gain the White House. His oldest brother, Joe Jr., died as a Navy pilot in 1944 while serving in Europe in World War II. President John F. Kennedy was assassinated in 1963 in Dallas. Sen. Robert F. Kennedy was killed by an assassin in 1968 in Los Angeles on the same evening he won California's Democratic primary for president.
"Senator Kennedy's death at the age of 77 reminds us of the extraordinary debt that our nation owes his family," said Richard Land, president of the Southern Baptist Ethics & Religious Liberty Commission. "His three older brothers all died at relatively young ages.
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"Senator Kennedy was the most liberal of the Kennedy brothers and, over more than four decades, was a passionate advocate for liberal causes in the United States Senate," he said. "However, he had many friends on both sides of the aisle, including some of the most conservative senators, because, while a fierce partisan politically, he was a loyal friend and friendly companion off the Senate floor.
"I had the privilege of working with him on several pieces of legislation which we both supported, including most recently the regulation of the tobacco industry," Land said.
Sen. Orrin Hatch, R.-Utah, a close friend of Kennedy, said in a written statement the senator's "influence cannot be overstated."
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Hatch, who worked with Kennedy to pass several significant bills, said, "[W]hile we almost always disagreed on most issues, once in a while we could actually get together and find the common ground, which is essential in passing legislation."
Pro-life and pro-family advocates normally found themselves opposing Kennedy's positions on social issues. After abortion was legalized by the Supreme Court's Roe v. Wade ruling in 1973, Kennedy fought to prevent restrictions on abortion rights, even opposing the Partial-birth Abortion Ban Act, which prohibits a gruesome procedure that involves the killing of a nearly total delivered baby. He also supported embryonic stem cell research and human embryo cloning for experimental purposes.
But Kennedy, a Roman Catholic, apparently had not always supported abortion rights. In a 1971 letter to a fellow Catholic reprinted on several Internet sites, Kennedy said it was his "personal feeling that the legalization of abortion on demand is not in accordance with the value which our civilization places on human life."
"When history looks back to this era it should recognize this generation as one which cared about human beings enough to halt the practice of war, to provide a decent living for every family, and to fulfill its responsibility to its children from the very moment of conception," Kennedy wrote.
One of Kennedy's sisters -- Eunice Kennedy Shriver, 88 -– was hailed by pro-lifers for her commitment to the unborn and to the disabled when she died Aug. 11. She opposed abortion before and after Roe and also founded the Special Olympics.
On homosexual issues, Kennedy sponsored the Employment Non-discrimination Act, which would include "sexual orientation" as a protected status in the workplace, and a hate crimes prevention bill that would encompass homosexual, bisexuals and transgendered people. He also supported "gay marriage" and was one of only 14 senators to vote against the Defense of Marriage Act in 1996.
In 1987, Kennedy led what proved to be a successful effort to defeat President Reagan's nomination of Robert Bork to the Supreme Court. If Bork had been confirmed, then Roe v. Wade may have been overturned in 1992, when the Supreme Court issued a ruling affirming abortion rights. Anthony Kennedy, the justice who was confirmed to the court after Bork's defeat, voted with the majority in that 5-4 decision.
Yet, Kennedy sometimes worked for legislation also supported by the Ethics & Religious Liberty Commission. Among such bills that became law were the following he either sponsored or was the lead Democratic co-sponsor for:
-- The Religious Freedom Restoration Act (RFRA), which was signed into law in 1993 in order to restore the previous understanding of religious free exercise but was invalidated by the Supreme Court, and the Religious Land Use and Institutionalized Persons Act, which dealt with two of the most important areas of free exercise and became law in 2000.
-- The Prenatally and Postnatally Diagnosed Conditions Awareness Act, which became law in 2008 and addressed the reported lack of information and support given to parents whose unborn children test positive for conditions such as Down syndrome.
-- The Family Smoking Prevention and Tobacco Control Act, which was signed into law in June and authorized the Food and Drug Administration to regulate tobacco products.
Only Sens. Robert Byrd, D.-W.Va., and the late Strom Thurmond, R.-S.C., served longer than Kennedy in the Senate. Byrd, who is still serving, has been in the chamber for 50 years. Thurmond served for 47 years.
It appeared early in Kennedy's political career he would be a likely presidential choice, but the death of a young woman in a 1969 auto accident in which he was the driver deeply damaged his chances. The senator drove off a bridge on Chappaquiddick Island at Martha's Vineyard in Massachusetts. While he escaped, his passenger, Mary Jo Kopechne, 28, did not and drowned. He failed to notify authorities for several hours.
Kennedy challenged President Carter for the Democratic nomination in 1980 but fell short.
Kennedy is survived by his wife, Victoria; three children, and two step-children. He also is survived by one sibling, Jean Kennedy Smith.
A special election will be held to fill Kennedy's seat, but state law mandates at least a 145-day wait.