Story highlights Political colleagues and foes describe Frank Lautenberg as a fighter

Lautenberg was a strong advocate for tougher gun laws

The New Jersey Democrat was the last World War II veteran in the Senate

Gov. Chris Christie permitted under law to appoint temporary replacement

Frank Lautenberg, a New Jersey Democrat associated with major legislative achievements over five terms in the U.S. Senate and the chamber's last surviving World War II veteran, died on Monday of viral pneumonia, his office said.

Lautenberg, 89, missed key Senate votes late last year during a weeks-long absence because of a cold that turned into what he called a "severe case of bronchitis with fluid in the chest." He also battled stomach cancer in recent years.

He announced in a statement in February that he would not seek re-election next year , but he continued to push for stronger gun control laws.

After last month's Boston Marathon bombings, Lautenberg said he would reintroduce legislation to require background checks for sales of explosive powder.

An influential businessman-turned-legislator, Lautenberg scored big victories in Congress, including a ban on smoking on airplanes, preventing domestic abusers from possessing guns, cracking down on drunken driving, and the "Toxic Right to Know" law about the release of pollutants into communities.

Photos: Frank Lautenberg's political career Photos: Frank Lautenberg's political career Frank Lautenberg through the years – Sen. Frank Lautenberg, D-New Jersey, passed away on Monday, June 3. Here, he presides over a hearing on Superstorm Sandy in Washington on December 6, 2012. Hide Caption 1 of 9 Photos: Frank Lautenberg's political career Frank Lautenberg through the years – President Bill Clinton and Lautenberg play golf on vacation in Martha's Vineyard on August 26, 1997. Hide Caption 2 of 9 Photos: Frank Lautenberg's political career Frank Lautenberg through the years – Lautenberg speaks with first lady Hillary Clinton on July 17, 2000, during a "Save America's Treasure" event on Ellis Island in New York. Hide Caption 3 of 9 Photos: Frank Lautenberg's political career Frank Lautenberg through the years – Lautenberg, right, meets with Sen. Tom Daschle, left, D-South Dakota, and Sen. Harry Reid, D-Nevada, on October 3, 2002. Hide Caption 4 of 9 Photos: Frank Lautenberg's political career Frank Lautenberg through the years – Senate candidate Frank Lautenberg, right, debates with Republican candidate Douglas Forrester at News12 New Jersey television studios October 30, 2002, in Edison, New Jersey. Hide Caption 5 of 9 Photos: Frank Lautenberg's political career Frank Lautenberg through the years – Lautenberg watches early voting returns with his grandchildren in New Brunswick, New Jersey, on November 5, 2002. Hide Caption 6 of 9 Photos: Frank Lautenberg's political career Frank Lautenberg through the years – Lautenberg attends a ceremony in honor of victims of the holocaust in Washington on May 1, 2008. Hide Caption 7 of 9 Photos: Frank Lautenberg's political career Frank Lautenberg through the years – Lautenberg discusses Juvenile diabetes with Nick Jonas in Washington on June 23, 2009. Hide Caption 8 of 9 Photos: Frank Lautenberg's political career Frank Lautenberg through the years – Lautenberg speaks to the press at the weekly Senate Democratic Policy Luncheon at the U.S. Capitol on January 29, 2013. Hide Caption 9 of 9

"Frank Lautenberg has been one of the most productive senators in the history of this country," Senate Majority Leader Harry Reid, D-Nevada, said in December 2011 on the occasion of Lautenberg's 9,000th vote, the statement by Lautenberg's office said.

On Monday, political colleagues and foes lauded Lautenberg as a passionate advocate for the issues he championed.

"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," said a statement by President Barack Obama, a former senator.

Secretary of State John Kerry, in remarks to reporters, called his longtime fellow Democratic senator "tenacious as a legislator" and someone who "never forgot where he came from."

In particular, Kerry cited Lautenberg's advocacy for environmental issues and his spirited defense of Kerry's military record when it came under attack by Republicans in the 2004 presidential election campaign.

"He was as loyal as he was patriotic," Kerry said, also noting how "during the years when Frank was going through chemotherapy and he was weakened by cancer, he was still down there on the floor of the United States Senate floor fighting on every environmental debate late into the night."

New Jersey's Republican governor, Chris Christie, told a GOP event on Monday that he often disagreed politically with Lautenberg, but he respected the senator's battling spirit.

"We had some good fights over our time -- battles on philosophy and the role of government," Christie said. "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. Senator Lautenberg fought for the things he believed in and sometimes he just fought because he liked to."

According to New Jersey law, Christie can appoint a temporary replacement to serve until a special election in November or fill the rest of Lautenberg's term, which expires next year.

With the likelihood that Christie will appoint a Republican, the Democratic majority in the 100-member Senate would decrease to 52, along with two independents who caucus with the Democrats.

Possible GOP appointees would include state Sens. Thomas Kean Jr. and Joe Kyrillos, a Christie ally who lost to Democrat Bob Menendez in the Senate race last year; Lt Gov. Kim Guadagno, and Assembly Republican Leader Jon Bramnick.

On the Democratic side, Newark Mayor Cory Booker announced in January he intended to run for the Senate next year, which caused a public tiff with Lautenberg, who later decided against seeking re-election.

The son of Russian and Polish immigrants, Lautenberg grew up in Paterson, New Jersey, and enlisted in the military at 18, his office said. He was a member of the United States Army Signal Corps from 1942 to 1946, according to his biography.

After the military, he graduated from Columbia University with the help of the GI Bill, then joined with two boyhood friends to start payroll processing firm Automatic Data Processing, or ADP, which grew into one of the world's largest computing services companies.

Lautenberg was first elected to the Senate in 1982 and won re-election twice. He did not seek re-election in 2000 when his third term expired, but was recruited to run again two years later after Democratic Sen. Robert Torricelli quit his re-election bid weeks before the polls opened amid reports of ethical issues. Lautenberg was then re-elected in 2008.

Survivors include his wife, Bonnie Englebardt Lautenberg; six children; and 13 grandchildren, the office statement said.