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Rep. Frank Pallone, D-N.J., received an important endorsement Monday in his bid for the U.S. Senate in New Jersey.

The Democratic congressman is running in a special Senate election to serve the remainder of the late-Sen. Frank Lautenberg's term and today he earned the backing of Lautenberg's family. In a clear dig to the frontrunner in the Democratic primary, Newark mayor Cory Booker, the family wrote that Pallone most reflects Lautenberg's values including his focus on being a "workhorse, not a showhorse."

"Frank Pallone, like Frank Lautenberg, has always looked out for working families in New Jersey and made them his top priority - in fighting for economic justice, health care reform, environmental protection, education and so many other issues," the letter reads. "Frank Pallone, like our Frank, will put in the hours and hard work necessary to fight for New Jersey in the Senate. And Frank Pallone knows that gimmicks and celebrity status won't get you very far in the real battles that Democrats face in the future."

It doesn't come as a surprise that the Lautenberg family would endorse Pallone over Booker because there were clear tensions between Lautenberg and Booker before his death. Most notably, Booker announced his intentions to seriously consider a run before the 89-year old Democratic senator had announced whether he would seek another term himself.

That didn't sit well with Lautenberg and he made it clear in no uncertain terms, saying Booker may be "entitled" to his seat, but comparing him to one of his badly behaved children.

"I have four children," Lautenberg told the Philadelphia Inquirer in January. "I love each one of them. I can't tell you that one of them wasn't occasionally disrespectful, so I gave them a spanking and everything was OK."

Lautenberg announced he would not run for another term in February and passed away last month.

The letter notes that the 14-term congressman and Lautenberg worked together "for many years" and he is the "best hope of continuing the progressive legacy" of Lautenberg. It also fits in another veiled jab at the headline-grabbing mayor.

"(Pallone) understands what it takes to take on and defeat Republicans and the special interests that attack the well being of working families," the letter reads. "While it may not always attract glamorous headlines, Frank knows that to be effective you must put New Jersey and your principles first, not your own glory."

A spokesperson for the Booker campaign responded to the family's letter telling ABC News in a statement: "Senator Lautenberg and Congressman Pallone worked together for decades in Congress, but ultimately, this race will be decided by which candidate best explains his or her vision for leading New Jersey and by who has a record of getting things done to improve lives."

Also important to note is that several top former aides of Lautenberg's who worked on previous campaigns of his are now working on the Booker campaign, including top political and communication advisers as well as the campaign's pollster.

Pallone and Booker, as well as Rep. Rush Holt, will face off in the Democratic primary in August and the special election will take place in October. The Holt campaigns did not immediately respond to requests for comment.

This story was updated with the Booker statement since it was originally posted.