In his more than 30 years in Washington, Leahy has become closely identified with some tough judicial battles over nominees to both the U.S. Supreme Court and federal appeals courts.
Of the nine sitting justices on the Supreme Court, Leahy has voted to confirm seven. He voted against conservative Justice Clarence Thomas' nomination in 1991 and opposed Samuel Alito's nomination in 2006.
During the nomination process for Supreme Court Justice John Roberts in 2005, Leahy talked to the NewsHour about the responsibility of the Senate in evaluating court nominees.
"It's not a responsibility to jump to an instant conclusion," he told Gwen Ifill. "It's a responsibility to take your time, really pay attention because we're representing all 280 million Americans. And I want to make sure that when I vote -- at this moment I have absolutely no idea how I will vote -- but I want to know when I vote I feel I've carried that responsibility."
Leahy surprised some when he voted to approve the nomination of Roberts to succeed the late Chief Justice William Rehnquist as the court's leader after peppering the then-nominee with tough questions during Senate hearings.
"I know this will not be popular with many of my constituency, and I understand that ... I came here to do what I thought was right, and as a Vermonter I can do nothing different, " he said of the vote, according to the Almanac of American Politics.
After news emerged that Supreme Court Justice David Souter planned to retire from the court in June 2009, Leahy told ABC's "This Week" that he would like to see more women on the court.
"Having only one woman on the Supreme Court does not reflect the makeup of the United States," he told the program in early May. "I think we should have more women, we should have more minorities."
Born in Montpelier, Vt. in 1940, Leahy attended Saint Michael's College in Colchester and graduated from Georgetown University Law Center in 1964. He served as a state attorney in Vermont's Chittenden County for eight years.
Leahy was elected to the U.S. Senate in 1974 at the age of 34. He is widely known as an outspoken Democrat, who in the weeks after the resignation of Justice Sandra Day O'Connor urged President Bush to be a "uniter, not a divider."
Leahy also is a member of the Appropriations Committee and serves on several subcommittees. In 1987, he was forced to resign from the Senate Intelligence Committee after allegations that he leaked sensitive information to an NBC reporter.
Leahy has been passionately involved in banning the production and export of land mines and, through his program, the Leahy War Victims Fund, provides up to $14 million per year in relief to landmine victims, his Web site reports. And, in recent years, he has been actively working to revise some provisions of the USA Patriot Act that he claims infringe on constitutional rights.
Leahy's also known to be an amateur photographer and a big "Batman" comics fan. He even had a brief speaking part in the 2008 Batman spinoff film "The Dark Knight."