Few doubted whom they were talking about.
On Friday, the Guardian newspaper's Glenn Greenwald, one of two reporters who broke the story about secret U.S. phone-spying programs, slapped at Booker on Twitter as a "Wall-St-controlled crony capitalism" candidate. Greenwald endorsed Rep. Rush Holt.
Earlier, Assembly Speaker Sheila Oliver, who is also running for the Senate, used Twitter to pointedly ask whether Booker would favor cuts to Medicare and Social Security.
Underlying the attacks are two of the left's biggest criticisms of the Newark mayor: his ties to Gov. Christie and to Wall Street.
"He's no Lautenberg. He's not a durable progressive," said Jay Lassiter, a liberal activist from Cherry Hill.
While Booker consistently leans left on some issues, such as same-sex marriage, Lassiter said that on others, the "slickly packaged" mayor appears malleable as he courts support ranging from Newark residents to the Napa Valley elite.
"A tremendous number of progressives in New Jersey are hungry for anyone who will engage them honestly and factually," Holt said earlier in the week.
He also touched on the contention that Booker's fame outpaces his work.
"They really are strengths, fund-raising and celebrity, but are these really strengths for the U.S. Senate?" asked Holt. "Playing to my strengths, I'm making this race about ideas."
Booker has campaigned as a uniter who has vowed to overcome the rancor in Washington, and the attacks don't seem to be damaging his prospects: A Monmouth University poll of likely Democratic primary voters last week showed him running nearly 40 points ahead of his closest competitor.
But his campaign bristled at accusations that he is too close to big financial institutions and that he is a celebrity. They say he has built his name on his record in Newark and used the resulting attention to help his city.
"Part of what's wrong with Washington is that the willingness to work across the aisle is often demonized," said Booker spokesman Kevin Griffis. "Mayor Booker is willing to work with anyone if it means improving the lives of the people he serves, and he's going to take that approach to Washington - a place in dire need of it."
New Jersey's most influential Democratic leaders have called Booker the party's best choice. While others poke at his stardom, party officials say Booker's name and fund-raising ability will help Democrats in future elections and give New Jersey added clout in the Capitol.
"You want to put your strongest player forward," State Senate President Steve Sweeney (D., Gloucester) said last month, "and there's no question Cory's the strongest player."
On federal issues, New Jersey leans strongly left, and on most of those topics, Booker has adhered to the Democratic position.
He backs President Obama's health-care law and has been an outspoken advocate for same-sex marriage. He supports abortion rights and has criticized Christie's decision to cut Planned Parenthood funding. He favors tougher laws on guns.
Booker's first policy rollout in the Senate campaign included a long list of Democratic dreams: providing universal preschool, raising the minimum wage, and expanding tax credits for people with low incomes.
He was a prominent surrogate for Obama last year on the campaign trail.
The mayor's aides also point to his personal story: a Rhodes scholar and Yale Law grad, Booker has worked for free legal clinics and moved to Newark to live in public housing. He used a stint living on a food-stamps budget to draw attention to poverty.
"The mayor's life's work has been fighting for the less privileged," Griffis said. "Those credentials are beyond question."
Compared with the rest of the Senate, any of New Jersey's Democratic candidates would sit securely on the left. The four primary contenders have more in common with one another than with moderate Democrats from states such as Montana or North Dakota.
But Booker has broken with party orthodoxy in several big ways.
He has supported charter schools and vouchers and has often stood by Christie, lending bipartisan credibility to the rising Republican star.
Booker backed Christie's pushes to change public pensions and teacher tenure, two of the governor's proudest moments (and anathema to labor unions), and disappointed Democrats by declining to run against the popular governor.
Booker last year defended Bain Capital on Meet the Press, even as Obama and his allies were using the firm to tar Mitt Romney.
Booker's comments led to a Republican Web page called "I Stand With Cory" and drew attention to his long-standing fund-raising from Wall Street.
While many New Jersey candidates receive money tied to the financial industry, others have taken more hostile stands. Holt has used two Web videos to attack big banks.
"I'm not tied to Wall Street," Holt said. "I'll let it go at that."
Defending Booker, Camden County Freeholder Ed McDonnell said supporting government reform helps progressive causes.
"To a real liberal, that's a positive. That means money and the opportunity to provide services people really need," McDonnell said when Booker visited Cherry Hill and Camden last week.
Statewide, Democratic voters have embraced the mayor. Among those who put a priority on "Democratic values," 49 percent support him, the Monmouth poll found.
Whatever their reservations, liberals will back Booker if he wins the primary, Lassiter said.
"In the end," he said, "we'll all come around."
Contact Jonathan Tamari at firstname.lastname@example.org or follow on Twitter @JonathanTamari. Read his blog, "Capitol Inq," at www.inquirer.com/CapitolInq.
Staff writer Andrew Seidman contributed to this article.