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The first big hint that Sen. Frank Lautenberg (D-N.J.) would not seek a sixth term in 2014 came in the form of a campaign-finance report: the Democratic incumbent raised about $11,000 in the final three months of 2012, a tiny fraction of what one might expect from a senator planning to run again.

But Lautenberg nevertheless showed a fighting spirit that kept people guessing -- even taking not-so-subtle shots at his would-be rivals -- until this afternoon, when the 89-year-old senator announced he'd step down at the end of his term.

"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 told the Star-Ledger. "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."

There's no great mystery about who intends to be his successor: Newark Mayor Cory Booker (D) kicked off his campaign several weeks ago, before Lautenberg had even made a public decision about his plans.

Polls suggest Booker is the early favorite to win the Democratic nomination, though it's worth noting that Rep. Frank Pallone (D) has also expressed interest in the race, as have some Democrats in the state legislature.

To date, no high-profile Republicans have expressed an interest in the race.

As for Lautenberg, a progressive champion for a generation, when he says he's on "a two-year mission to pass new gun safety laws," the senator means it. Arguably no one in the chamber is as fierce an advocate of reducing gun violence as Lautenberg, and I suspect he'll invest all of his energies into the issue between now and the end of next year.