If you haven't heard much about Cory Booker lately, it's no accident.

The Newark mayor — typically at home in the spotlight, at the center of a crowd, or on the set of a cable news show — spent much of April behind closed doors, working to formalize the nascent stages of his bid for U.S. Senate, professionalize his campaign operation, create an early fundraising advantage, and let the people of New Jersey focus on the gubernatorial race this fall.

Aides to the mayor, who won't make his bid for Senate official until after the governor's race, say privately that Booker is laying low, and even avoiding big interviews, to buckle down on creating a foundation for his campaign and concentrate on his last 400 days in Newark City Hall.

It's a quiet but calculated effort to do what doesn't come naturally to America's favorite mayor: Stay out of the news.

During last year's presidential campaign, Booker was a prominent surrogate for President Barack Obama. He was also a regular fixture on CNN and MSNBC, where he continued to appear even after Election Day, first to promote his "SNAP Challenge" in December — when Booker lived for a week on meals provided by a food stamp budget — and later to address speculation over whether he would run for governor or Senate. But since then, Booker has been largely absent from network and cable news shows. He gave six television news interviews in December, five in January, four in February, three in March — and in April, Booker did not appear on a single television news program, sticking instead to his monthly radio interview on WBGO's "Newark Today" show.

His recent public appearances, too, have not been the kinds that make news among the political class. In the last month, Booker's events have ranged from local outings in Newark — a press conference for the city's gun-buyback program, or an exhibit opening at the Newark Museum — to social media–themed conferences and panels, where Booker is asked by tech-industry moderators to discuss his digital media company, Waywire, and only sometimes answer a question or two about his Senate campaign.

"He's hunkering down," said Doug Muzzio, a Baruch College political science professor and longtime observer of New Jersey politics. "He's raising money, and he's putting together a staff. He's doing all the things that you need to do to be a serious candidate."

"The problem is that Twitter alone does not win campaigns," said Democratic strategist Hank Sheinkopf, referring to Booker's following of 1.4 million on the social media site. "He is now figuring out what kind of structure to have in place, who's going to be working for him, what kind of consultants to have — when he figures those things out, he'll have a real campaign."

Following the departure of three campaign staffers at the start of the year, Booker hired three new aides who have since helped add structure to his occasionally loose, on-the-fly operation: Finance Director Lauren Dikis, who worked previously at the Democratic Senatorial Campaign Committee; Campaign Communications Director Kevin Griffis, a former Obama staffer; and City Hall Communications Director James Allen, who came from Capitol Hill to fill the position after almost a year of it being vacant.

Before the new team was assembled, Booker may as well have been his own communications director — the mayor would often tweet directly at reporters or make calls for interviews from his personal cell phone. That has certainly changed.

For an interview last week with BuzzFeed about New Jersey's Democratic candidate for governor, whom Booker supports, Griffis arranged a conference call — complete with a dial-in code and password — so both he and Booker could be on the line at the same time. As the interview came to a close and Griffis pronounced the call over, Booker obediently ignored an appeal for another question. "My new communications director would kill me," he joked.

Booker has also gotten serious about raising cash for the effort in advance of a possible primary challenge from other New Jersey Democrats. He has held at least 10 fundraisers inside and outside the state since March in support of the Senate bid, traveling most recently to Chicago Tuesday evening, on the heels of what was likely a fruitful swing through California — to San Francisco for an event at a prominent law firm, and to Los Angeles for a private, $5,000-per-head reception at the home of major motion-picture producer Jerry Weintraub.

In the first quarter alone, before most of his major fundraisers, Booker netted more than $1.9 million, even with a late start — he didn't file paperwork with the FEC to create a campaign committee until Jan. 8.

With the governor's race underway between incumbent Republican Gov. Chris Christie and challenger Democratic state Sen. Barbara Buono, Booker has the time and space to operate quietly while the rest of the state eyes the contest this November.

The governor's race, too, provides a crucial opportunity for the mayor to reconnect with state Democrats. Booker is no doubt a national star, but inside New Jersey, his willingness to operate outside the state party apparatus has nettled his peers in the political class before.

Last December, Booker's decision to run for Senate instead of governor came after a long, public period of deliberation that Democrats said held up other potential gubernatorial candidates from starting their own campaigns, and riled the 89-year-old Sen. Frank Lautenberg, whose seat Booker said he wanted before the senior senator had even announced he was retiring. (Lautenberg finally did about two months later.)

"I know he rubbed people the wrong way with the way he handled it," said a New Jersey Democratic operative. "But the governor's race gives him a chance to show a little humility and be a good soldier this year."

"He angered some people when he came out of the gate that quickly and talked about running against Lautenberg," Sheinkopf said. "In New Jersey, that kind of behavior is remembered."

By virtue of his name recognition alone, Booker has already proven a valuable asset to Buono, who is unknown inside the state relative to Christie. He has held one event for the candidate in Newark, sent a campaign email on her behalf, and plans to host a fundraiser for her early this month. And Buono told BuzzFeed last week that she couldn't be happier with Booker's level of involvement.

"He has been wonderful. He told me, 'There's no such thing as over-asking.' Direct quote," said Buono, who is trailing Christie by a 30-point margin in the polls. "Whatever we want he said he'd do. Obviously fundraising is big. But he validates that the campaign is winnable. It's important that you have validators."

In an interview on MSNBC last month, when asked about Facebook founder Mark Zuckerberg's support for Christie — the media mogul held a fundraiser for the incumbent governor earlier this year in California — Buono said simply, "He may have Mark Zuckerberg, but I have Cory Booker."

Booker also appeared last month at a private-home fundraiser to endorse Dave Haas, the Democratic candidate for mayor in Westfield, New Jersey. Helping his colleagues in the party — and crisscrossing the state for Buono, as he has vowed to do — will certainly help Booker shore up support between now and next year from state party leaders.

"I'm not doubting his motives, and I think he wants Barbara to be the next governor of the state," said one New Jersey Democratic official. "But this makes a whole lot of sense for him. He tripped on some of his politics in terms of the rollout of his campaign, or whatever you want to call that, so he'll now have to do a bit of work."

But even if Booker faces a serious primary challenge from Reps. Frank Pallone and Rush Holt, the two candidates said to be considering a bid for the Senate seat, the Newark mayor will likely have a serious advantage in both name identification and fundraising. A Fairleigh Dickinson University poll shows that 50% of state Democrats would vote for Booker in the race, while only 7% would vote for Holt, and just 4% for Pallone. That's the sort of combined advantage, in money and in popularity, that could just scare off a primary challenger.

Democrats add that Pallone, who has reportedly been interested in Lautenberg's seat for years, may opt to stay out of the race to keep his spot as a ranking member of the House Energy and Commerce Subcommittee on Health.

"Pallone is telling people he wants to run, and he's all over the state, showing up at county dinners and making small changes to his staff," said the Democratic state official. "But it's a big risk for him. If he really wanted to run, it would require him being very forceful about what his intentions are. And so far I've just seen a dip-his-toe-in-the-water approach."

And until another candidate gets in the race, Booker is likely to stay in his newfound spot behind the scenes, and away from the spotlight.

"All he has to do right now is raise money and not anger anybody by moving too quickly," said Sheinkopf.