All eyes now will turn to the Democratic primary to succeed him, since Republicans have virtually no bench here and little hope of winning a Senate seat in the Garden State. (The GOP hasn't won an election for Senate in New Jersey since 1972.) Booker had taken several steps toward a primary challenge to Lautenberg, but in so doing, he seemed to infuriate much of the Jersey Dem establishment by not showing sufficient respect to the incumbent. They may secretly be glad Booker wasn't willing to wait his turn in line, though, since his impatience may have helped usher Lautenberg out the door, and now other Democrats will feel free to jump in.
The person who's seemed closest to a bid is Rep. Frank Pallone, who, according to various reports, has been slowly and quietly locking up support from the various local power-brokers who are often crucial to success in New Jersey politics. Other possibilities include state Senate President Stephen Sweeney, Assembly Speaker Sheila Oliver, and even Rep. Rob Andrews, who ran a disastrous primary campaign against Lautenberg in 2008. State Sen. Richard Codey could also be a possibility. Booker, incidentally, never officially declared that he'd run, but it would be pretty surprising if he now decided to pass up the chance.
New Jersey hasn't had an open Senate seat since 2000, which incidentally was the last time Lautenberg retired before his unexpected comeback in 2002, so we can expect a pretty intense contest, even if it's just a one-on-one matchup between Booker and Pallone. And if it's a battle between a young self-styled reformer with ambitions of national profile versus a creature of the old school intent on following the traditional playbook, it could be a very interesting race indeed.