Rep. Rush Holt, left, says Mayor Cory Booker needs to show more candor. (Getty Images)

Rep. Rush Holt, D-N.J., says Newark Mayor Cory Booker has "some explaining to do" about why his start-up firm Waywire quietly employed the 15-year-old son of CNN President Jeff Zucker as a board member paid with stock options.

Holt and Booker are competing for votes ahead of the Garden State's Aug. 13 Democratic primary to pick a successor to Sen. Frank Lautenberg, who passed away earlier this year.

"I don't think it takes a lot of thinking to see what's going on there," Holt told U.S. News in a Thursday phone interview.

"I read that in The New York Times yesterday and my jaw dropped," he said. "Mayor Booker has some explaining to do."

Andrew Zucker's position on the board of Waywire was revealed by the Times just six days ahead of the election that Booker is favored to win.

Waywire was founded by Booker, who reportedly serves as chairman, and two colleagues in 2012 to "collect, curate and share" videos. It has received funding from Google Chairman Eric Schmidt and TV host Oprah Winfrey.

A company spokesman told Fortune magazine that Booker was not directly involved in hiring Zucker's teenage son, who abruptly resigned his position Thursday afternoon.

It's unclear if the boy will retain his stock options, but Holt said that may not matter because "the company doesn't seem to be going anywhere." Indeed, the video service's website reportedly had just 2,207 visitors in June.

Kevin Griffis, a spokesman for Booker's Senate campaign, told U.S. News the popular mayor "is not involved in the day-to-day operations" of Waywire.

Holt, an eight-term congressman, is angling for a last-minute surge in voter interest. His Senate campaign is focused on pushing progressive policies like single-payer health care, the implementation of a carbon tax and breaking up large banks.

He is also a vocal critic of the NSA's collection of phone and Internet records, and proudly voted for the Amash amendment last month to defund the bulk collection of phone metadata. Holt says Booker hasn't made his own position clear on the issue.

"Mayor Booker, the person the media anointed as the winner a month or two ago, his campaign has been generally devoid of issues," Holt told U.S. News. "If a candidate is not forthcoming, it's hard to know how the candidate will behave if he ever gets into office."

Rep. Frank Pallone is also contesting next week's Democratic Senate primary. A poll conducted Aug. 1-5 by Quinnipiac found Booker the heavy favorite. The general election will be Oct. 16.