Frank Lautenberg, pictured here in 2006, will not run for another term. (AP Photo/Charles Dharapak) Frank Lautenberg, pictured here in 2006, will not run for another term. (AP Photo/Charles Dharapak)

Sen. Frank Lautenberg (D-N.J.) will not run for reelection, he announced Thursday, ending five terms in the Senate.

“I am not announcing the end of anything. I am announcing the beginning of a two-year mission to pass new gun safety laws, protect children from toxic chemicals and create more opportunities for working families in New Jersey,” the 89-year-old senator said in a statement. “While I may not be seeking re-election, there is plenty of work to do before the end of this term and I'm going to keep fighting as hard as ever for the people of New Jersey in the U.S. Senate.”

Lautenberg quipped to reporters Thursday that he had decided to announce his retirement because, “I looked out, and it’s time for me to be mature."

A self-made millionaire and the son of Eastern European immigrants, Lautenberg is the last U.S. military veteran of World War II serving in the Senate. He first retired from the Senate after three terms in 2000, a decision he later said that he regretted. A scandal involving Sen. Robert Torricelli (D-N.J.) gave Lautenberg an easy opening to return to the Senate in 2002, where he emerged as a vocal critic of the Iraq War. He won reelection for a fifth term in 2008.

Sen. Bob Menendez (D-N.J.) -- who is facing a Senate Ethics Committee investigation and some calls to step down -- praised his Garden State colleague.

“He’s been a fighter for New Jersey, he’s had some tremendous successes on behalf of the state, national legislation of tremendous import, and I’m sure he’ll spend the next two years continuing to make that kind of impact for the nation and New Jersey," Menendez told reporters Thursday.

Lautenberg's decision opens the door for Newark Mayor Cory Booker (D) to run for the seat after months of openly contemplating a campaign. That talk has irked Lautenberg, who once again needled the mayor a bit Thursday.

"There's work to be done in Newark, and Cory Booker will have to enlighten us," Lautenberg told reporters. "It's so funny, because my office is in Newark, and every day that I go to work, I go to Newark. Everyday I go to work, he leaves."

“I’d encourage him to finish the job that has to be done in Newark," Lautenberg said of Booker. "We still have a lot of violence, we still have a lot of inconvenience. My office is across the street from Penn Station there, and if I try to leave the building at 6 o’clock at night, sometimes it takes 20 minutes to get out of the parking lot. Newark is a place that deserves encouragement, it’s our largest city and [it has] many education problems.”

Despite Lautenberg's displeasure, polls have routinely shown Booker leading the senator in a hypothetical primary, while many voters in the Garden State see Lautenberg's advanced age as a liability. A Monmouth poll today showed Booker ahead of Lautenberg 40 percent to 25 percent.

Booker said in a statement Thursday that Lautenberg "has been a champion for the people of New Jersey for decades and his legacy of service will improve the lives of all American's for years to come. On a personal note, Senator Lautenberg has been a strong model of leadership and service to me since before I even considered entering elected office."

With Lautenberg getting out, the question now is whether Booker will face significant opposition for the Democratic nomination. There's no shortage of ambitious Democrats in New Jersey, most of whom passed on challenging Gov. Chris Christie (R).

Rep. Frank Pallone (D-N.J.) "is running," according to a Democrat with knowledge of his plans. In a statement, Pallone said he was "proud to serve with Senator Lautenberg and even prouder to call him a friend." Observers say that State Senate President Steve Sweeney (D) is also thought to be interested.