"Most of the candidates in the Democratic field have proven themselves as hardworking, progressive leaders who care deeply about New Jersey. But only one of them stands out as ready to continue Frank Lautenberg's progressive leadership in the U.S. Senate. That candidate is Congressman Frank Pallone."
"We are saying: Stick with Frank," they added.
Lautenberg, who died in June, had accused Booker of spending too much time outside New Jersey building a national profile rather than focusing on Newark, where he serves as mayor.
Booker has become a political star with frequent TV appearances, a prominent role in President Obama's campaign, and eye-catching heroics that have made national headlines, including shoveling out constituents during a snowstorm, rushing into a burning home to rescue a neighbor, and living on the equivalent of foods stamps for a week.
The Lautenberg announcement came as Booker appeared in New York on ABC's Live With Kelly and Michael.
The family statement was issued by the senator's widow, Bonnie, and his four children, and prepared by them, the Pallone campaign said.
"Frank Pallone, like our Frank, will put in the hours and hard work necessary to fight for New Jersey in the Senate," the statement said.
It pointedly continued: "Frank Pallone knows that gimmicks and celebrity status won't get you very far in the real battles that Democrats face in the future . . . 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."
The endorsement is the most positive development yet for Pallone as he runs in a four-way Democratic primary in a race to replace Lautenberg, who held a New Jersey Senate seat for much of the last 30 years.
Support from Lautenberg's family could help Pallone distinguish himself from fellow Rep. Rush Holt, who also is running as a liberal champion and has argued that he would be the best choice to carry on Lautenberg's legacy. His and Pallone's similar profiles have been seen as a drag on their candidacies.
The fourth Democrat in the race is Assembly Speaker Sheila Oliver.
Pallone was close with Lautenberg; before he took office himself, Pallone volunteered on Lautenberg's first campaign, in 1982.
"Sen. Lautenberg and Congressman Pallone worked together for decades in Congress, but ultimately, this race will be decided by the voters and by which candidate best explains his or her vision for improving the lives of New Jerseyans," said a Booker campaign statement.
Booker has won support from most of the Democratic establishment and has hired a few of Lautenberg's former campaign aides, but he angered Lautenberg when he openly made moves toward running for Senate before Lautenberg announced his decision on seeking reelection.
In January, Lautenberg suggested "spanking" Booker for disrespect.
Lautenberg eventually announced he would not run in 2014. He died in office in June, setting the stage for an Aug. 13 primary and Oct. 16 special election.
Contact Jonathan Tamari at firstname.lastname@example.org or follow on Twitter @JonathanTamari. Read his blog "Capitol Inq" at www.inquirer.com/CapitolInq.
This article contains material from the Associated Press.