Kevin Griffis said Booker was stepping down "to remove even the perception that he would be distracted from his duties as senator," adding that "voters can be assured that the mayor will not derive profit from his stake in the company."
Booker, 44, reported total income of $512,728 in 2012 and paid $115,663 in taxes - for an effective rate of 36.9 percent on taxable income of $313,560. He has made $3.9 million since 1998, the returns show, largely through public speaking fees and his work for a Newark law firm to supplement his income on Newark City Council and as mayor.
Broadly, the returns show that Booker's income has risen steadily over the last decade - along with his star in the Democratic Party. He has paid more than $1 million in taxes since 1998, the returns show.
"Cory Booker has now made a historic gesture of transparency, and we challenge Steve Lonegan to do the same," Booker campaign manager Addisu Demissie said in a statement.
A spokesman for the Lonegan campaign said the candidate, a former mayor of Bogota in Bergen County, already had released his tax returns to the New York Post and would make them available Monday for other news organizations.
In the latest poll, an Aug. 29 Fairleigh Dickinson Public Mind survey, Booker led Lonegan by 28 points. The special election is Oct. 16.
The tax returns were made available for three hours at Booker's campaign headquarters to select news organizations on the grounds that the returns not be copied or photographed.
Booker's involvement in Waywire, a video-sharing service, has generated controversy. The New York Times reported in early August that Booker had the largest stake in the company - from $1 million to $5 million - even though he was not involved in day-to-day operations.
Some of the entrepreneurs financing Waywire have also donated to Booker's campaign.
Lonegan blasted Booker last month as "beholden to the Silicon Valley billionaires who funded his 'start-up' business."
Candidates for Senate are not required to release tax returns, and members of Congress file only annual financial disclosure forms.
"Throughout his career in public service, Mayor Booker has worked to make government more transparent and accountable," Demissie said, "and in his race to become New Jersey's next U.S. senator, he has again set the bar."
Booker also released his 2002 returns during his first and unsuccessful bid for mayor of Newark.
Booker's earnings have grown from his salary of nearly $41,000 as a city councilman in 1998 to more than $400,000 each year since 2008.
He stepped down from the West Orange law firm Trenk DiPasquale when he became mayor in 2006. He has received separation payments from the firm totaling $688,500 since 2007, according to the returns.
Those buyout payments have also come under scrutiny because of the firm's contracts with the City of Newark.
"Cory Booker has received massive six-figure payouts following a brief association with a law firm that has received millions of tax dollars from city agency contracts while he has been mayor," said Lonegan spokesman Will Gattenby.
Through public speaking alone, Booker has amassed $1.3 million since 2000. He has donated $149,347 of that money.
Other sources of income include $25,026 for a teaching fellowship at Rutgers in 2003.
Booker has had mixed success with his investments. In 2008, he reported a capital gain of $79,686 after he sold 3,981 shares of K12 Inc., a for-profit education company. He acquired the shares in April 2006.
He also reported $9,968 in capital gains in 2006 from the sale of his investment in American Tower, a communications infrastructure company.
But while Booker has a clear interest in the Internet - in addition to his role with Waywire, he famously has 1.4 million followers on Twitter - he may have underestimated the value of one industry-changing firm.
In 2003, he made $172 by selling 25 shares of Netflix. Shares of the Internet-streaming company closed Friday at $291.54.
Contact Andrew Seidman at 856-779-3846 or email@example.com, or follow no Twitter @AndrewSeidman.