In this Jan. 28, 2013, file photo, Sen. Frank Lautenberg, D-N.J., speaks to reporters after the Senate passed a $50.5 billion emergency relief measure for Superstorm Sandy victims at the Capitol in Washington. (J. Scott Applewhite/AP)

Sen. Frank R. Lautenberg, a self-made multimillionaire and the only U.S. military veteran of World War II still serving in the U.S. Senate, will not seek reelection in 2014.

The New Jersey Democrat, 89, made his decision official Thursday after months of speculation in his home state and Washington as Newark Mayor Cory Booker (D) began openly discussing the possibility of running for the seat.

“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,” Lautenberg said in a statement.

Asked later about a Booker candidacy, Lautenberg criticized the mayor for seeming more concerned about his growing national profile than the fate of New Jersey’s largest city.

“It’s so funny, because my office is in Newark and every day that I go to work, I go to Newark. Every day I go to work, he leaves,” Lautenberg quipped, adding later that “I’d encourage him to finish the job that has to be done in Newark. We still have a lot of violence, we still have a lot of inconvenience.”

A Monmouth University poll released Thursday showed Booker leading the senator 40 percent to 25 percent in a hypothetical matchup, while many voters in the Garden State consider the senator’s age a liability.

Booker in a statement said 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 Americans 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.”

After serving in Europe during World War II, Lautenberg graduated from Columbia University with assistance from the G.I. Bill and co-founded the nation’s first payroll services company, Automatic Data Processing. By the 1990s, the company processed almost 10 percent of private-sector paychecks in the United States — and Lautenberg was worth at least $40 million.

On Thursday, President Obama lauded Lautenberg’s time in the military, calling it “a testament to his character and deep commitment to public service.”

With Lautenberg out, the question now is whether Booker will face significant opposition for the Democratic nomination. A slate of ambitious Democrats in New Jersey mostly passed on challenging Gov. Chris Christie (R) in this year’s gubernatorial race. Rep. Frank Pallone Jr. (D) and State Senate President Stephen Sweeney (D) are said to be interested in a potential Senate race, according to Democrats with knowledge of their plans.

Lautenberg served in the Senate from 1982 to 2001 and returned 2003 after a brief retirement.

He is the fourth senator to announce plans not to seek reelection, joining John D. Rockefeller IV (D-W.Va.), Tom Harkin (D-Iowa) and Saxby Chambliss (R-Ga.).

Rachel Weiner and Aaron Blake contributed to this report.