Democratic New Jersey Sen. Frank Lautenberg, who at 89 was the oldest member of the Senate and was its last remaining World War II veteran, died Monday, his office said.

Lautenberg died at New York-Presbyterian Hospital due to complications from viral pneumonia.

The Paterson-born lawmaker had announced in February that he wouldn't seek re-election next year.

Lautenberg was first elected to the Senate in 1982 -- which was his first run for public office -- and was re-elected in 1988 and 1994. He briefly retired and then successfully ran for office again in 2002, followed by a re-election in 2008, making him the first New Jersey lawmaker ever elected to five Senate terms.

At the time, the 84-year-old dismissed concerns about his age.

"People don't give a darn about my age," he said. "They know I'm vigorous. They know I've got plenty of energy."

Lautenberg was last on the Senate floor April 17, when he received an ovation from his fellow senators as he arrived in a wheelchair to vote for a gun control measure.

He cast his 9,000th vote in December 2011, and at the time, Senate Majority Leader Harry Reid said "Frank Lautenberg has been one of the most productive senators in the history of this country."

Reid said in a statement Monday that "as a senator, Frank never compromised his principles and was a fierce advocate for the citizens of New Jersey."

"Millions of Americans are healthier and safer because of legislation he championed," Reid said.

In the 1980s, Lautenberg was a driving force behind the laws that banned smoking on most U.S. flights and made 21 the drinking age in all 50 states.

"He was someone who fought and won a lot of battles that today people just take for granted," said Secretary of State John Kerry, who served in the Senate with Lautenberg for more than 20 years.

President Bill Clinton and his wife, Hillary Clinton, said in a statement that "New Jersey has lost a favorite son, the country has lost a great legislator, and we have lost a dear friend and cherished colleague."

Lautenberg was a reliable vote for traditional Democratic policies, though he bucked President Clinton in 1993 on the budget because he said it raised taxes and didn't cut spending enough. He also voted against Clinton on the North American Free Trade Agreement, opposed by the staunch labor allies Lautenberg had come to depend on.

Later in his career, he became a foil for Christie.

In 2012, Christie called Lautenberg a "partisan hack" and an "embarrassment" and said it was time for him to retire. Lautenberg called Christie "the name-calling governor" and, in one speech, "the king of liars."

Lautenberg, who enlisted in the Army when he was 18, served in Europe during World War II. He founded a company, Automatic Data Processing, and then left to run for office.

Lautenberg is survived by his wife, six children and 13 grandchildren.

Newark Mayor Cory Booker, who had announced plans to seek the seat next year, said in a statement Monday that "the American people lost a true champion."

"For three decades, Sen. Frank Lautenberg worked to make America a stronger, healthier and safer place to live. His legacy will endure for generations," he said.

Republican Gov. Chris Christie will appoint a senator to fill the seat, and a special election will determine a replacement.