Sen. Frank Lautenberg, D-N.J., passed away from viral pneumonia at the age of 89.

New Jersey Sen. Frank Lautenberg passed away on Monday after struggling with health issues in recent months.

NBC News confirmed that the 89-year-old Democratic senator – the oldest member of the chamber – passed away from viral pneumonia that had sidelined him from the Senate for much of this year.

President Barack Obama hailed the late senator as "a proud New Jerseyan who lived America’s promise as a citizen, and fought to keep that promise alive as a senator."

Lautenberg was first elected to the Senate in 1982, and served into that capacity through 2001, when he initially retired. State Democrats beckoned him back into public life, though, in 2002 following the withdrawal of their candidate during a competitive race.

"First elected to the Senate in 1982, he 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," Obama said.

Though Lautenberg missed a number of votes over the past few months due to illness, he did return to Washington for several high-profile votes. He made the trip to the Capitol in April for the Senate’s vote on legislation to strengthen gun control laws, and on May 16, he participated in a committee vote to approve President Barack Obama’s nominee to head the EPA.

"Never was Sen. Lautenberg to be underestimated as an advocate for the causes he believed in and as an adversary in the political world," New Jersey Gov. Chris Christie, a Republican, told the Philadelphia Inquirer. "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 am deeply saddened at the loss of my friend, Frank Lautenberg," said Senate Majority Leader Harry Reid, D-Nev. "History will show him to be one of the most productive Senators ever."

The longtime senator was the last remaining member of the Senate to have served in the military during World War II, following the retirement earlier this year of Sen. Daniel Akaka, D-Hawaii, and the death of Sen. Daniel Iouye, D-Hawaii. The flags at the U.S. Capitol were ordered to half-staff on Monday in mourning of Lautenberg.

Chip Somodevilla / Getty Images NBC News has confirmed that Sen. Frank Lautenberg, D-N.J., the oldest member of the Senate, has died at the age of 89 of viral pneumonia June 3, 2013.

Lautenberg had announced his intention to retire in 2015, at the conclusion of his current term -- though not before publicly chastising Newark Mayor Cory Booker, D, for readying his own Senate campaign before Lautenberg made that announcement. The task will now fall to Christie to appoint an interim successor to Lautenberg.

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