(CBS/AP) Sen. Frank Lautenberg, D-N.J., the last World War II veteran serving in the Senate, died due to complications from viral pneumonia at 4:02 a.m. Monday at New York-Presbyterian Hospital/Weill Cornell, his office announced. He was 89 years old.
The Democrat had health problems in recent years and had missed several Senate votes in the first months of the year. He had the flu and missed the Senate's Jan. 1 vote to avoid the so-called fiscal cliff of rising taxes and falling government spending, then missed several votes two months later because of leg pain.
A chest cold kept him from attending a May 29 tribute in New York honoring him for his contributions to the Jewish community and Israel.
He had been diagnosed in February 2010 with B-cell lymphoma of the stomach and underwent chemotherapy treatments until he was declared in June 2010 to be free of cancer. He worked between the treatments. The diagnosis came just days after the death of West Virginia's Robert Byrd made Lautenberg the oldest member of the Senate.
Before going to Washington, he was a successful businessman who co-founded the firm Automatic Data Processing.
Lautenberg is survived by his wife, Bonnie Englebardt Lautenberg; six children and their spouses and 13 grandchildren.
Lautenberg announced in February that he would not seek reelection next year, opening the door for Newark's Democratic Mayor Cory Booker and others to run for the Senate seat.
Lautenberg's death creates a vacancy that Republican Gov. Chris Christie will fill temporarily. According to New Jersey election law, Christie will appoint someone to the seat until a special election is held.
The law states that a special election is to be scheduled when the next general election is held in the state, or on another date, if Christie chooses. If he elects to schedule it concurrent with the next general election in New Jersey, it will be on the same date as the gubernatorial election this November (Christie is running for reelection this year).
However, there's conflicting language in the statute which may allow Christie to wait for the November 2014 election, when the full six-year term for this seat is scheduled. If he follows that path, then whoever Christie appoints could have a 17-month incumbency advantage and, if he chooses a Republican as interim senator, will surely stir up controversy in the heavily-Democratic Garden State.
Christie, who frequently sparred with Lautenberg - he once called Christie "king of the liars" and last year Christie called Lautenberg "a waste of my time" - pointed out the obvious at a press conference Monday, saying that he and the late senator didn't usually agree.
"It's no mystery that Senator Lautenberg and I didn't always agree," Christie said. "In fact, it probably is more honest to say we very often didn't agree, and we had some pretty good fights between us over time."
Despite their differences, Christie did pay his respects:
"I think the best way to describe Frank Lautenberg - and the way he would probably want to be described to all of you today - is as a fighter. Sen. Lautenberg fought for the things he believed in, and sometimes he just fought because he liked to."
"I give him praise for a life well-lived," added Christie.
President Obama said in a statement, "Michelle and I were deeply saddened to learn of the passing of Senator Frank Lautenberg, a proud New Jerseyan who lived America's promise as a citizen, and fought to keep that promise alive as a senator.
"[H]e improved the lives of countless Americans with his commitment to our nation's health and safety, from improving our public transportation to protecting citizens from gun violence to ensuring that members of our military and their families get the care they deserve."
In the Senate, Lautenberg was a longtime advocate of Amtrak and other transportation causes, as well as environmental, health care and veterans' issues. Lately, he had helped lead efforts in the Senate to craft gun control measures, introducing a bill to ban high-capacity ammunition magazines holding more than 10 rounds.
Lautenberg was a staunch gun control advocate and frequent critic of the tobacco industry, and he fought for greater government spending on transportation and the environment. He wrote the laws banning smoking on domestic airline flights and setting the national minimum drinking age of 21.
Along with Lautenberg's legislative accomplishments, he had a string of electoral coups, including an upset over someone he called "the most popular candidate in the country" in his first race for Senate, and a victory in a strange, abbreviated, back-from-retirement campaign 20 years later.
He served nearly three decades in the Senate in two stints, beginning with an upset victory in 1982 over Republican Rep. Millicent Fenwick, the pipe-smoking, pearl-wearing patrician who was the model for the cartoon character Lacey Davenport in "Doonesbury."
He initially retired in 2000 after 18 years in the Senate, saying he did not have the drive to raise money for a fourth campaign. He served on the boards of three companies, two graduate schools and the U.S. Holocaust Memorial Museum.
But New Jersey Democrats recruited Lautenberg out of retirement in September 2002 as an 11th-hour replacement for Robert Torricelli, Lautenberg's longtime rival, who had abandoned his re-election bid just five weeks before Election Day.
Republicans went to court to prevent what they called the Democratic Party's ballot "switcheroo." When that failed, they attacked Lautenberg as a political relic ill-suited for dangerous times.
But Lautenberg surged to an easy win over Republican Douglas Forrester and returned to the Senate in 2003 at age 78, resuming his role as a leading liberal, and he made it clear that his return to office was no mere cameo.
When Democrats regained a Senate majority in 2007, he returned to the powerful Appropriations Committee, on which he had served for 15 years.
At age 84, he beat back a Democratic primary challenge in 2008 and went on to another easy win in the November general election. It made him the first New Jersey person ever elected to five Senate terms.
"People don't give a darn about my age," Lautenberg said. "They know I'm vigorous. They know I've got plenty of energy."
© 2013 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.