A powerful Democrat from the northern half of the state, state Sen. Ray Lesniak, also threw his support behind Andrews.
Roberts was among scores of New Jersey Democratic bigwigs who stood alongside Lautenberg as he announced his bid for re-election this week.
"I'm as happy as I can be to express my support for Frank Lautenberg," Roberts said during Monday's event. "Every single time there's been an issue that has been of importance to South Jersey, Sen. Lautenberg has been there."
Roberts spokesman Michael Muller says that Roberts' view of the race changed when Andrews entered it Wednesday, 48 hours after Lautenberg formally declared his intention to run again at age 84.
Andrews, 50, has represented South Jersey's 1st Congressional District since 1990. Roberts represents Camden County in the New Jersey Assembly.
In a letter to Andrews, Lesniak praised Lautenberg but said it is time for a change.
"Congressman, we need your vision, dedication and energy to end the war in Iraq and bring out troops home," said the letter.
It was co-authored by a fellow Union County Democrat, Sen. Nicholas Scutari. The endorsement by senators from Union County adds weight to Andrews' bid to unseat Lautenberg.
In another blow to the Lautenberg campaign, the New Jersey AFL-CIO retracted its endorsement.
In a statement issued Thursday, the endorsement was "inaccurate due to a lack of relevant information."
The union requires a two-thirds vote of its executive board to allow an endorsement in a primary. Lautenberg got the required two-thirds when polled against Tom Byrne, the son of former Gov. Brendan Byrne. But he did not get the required number of votes when polled against Andrews. Byrne decided not to run.
With no clear choice from the state AFL-CIO, affiliates can support their candidate of choice, the statement says.
Andrews also has the backing of the two Middlesex County state senators, Barbara Buono and Joseph Vitale, and of Essex County powerbroker Steve Adubato Sr.
Andrews is widely unknown in the northern half of the state, compared to Lautenberg, a liberal Democrat who has been in the Senate almost continuously since 1983.
A virtually unknown Democrat, Morristown Mayor Donald Cresitello, has also indicated he would enter the June Democratic primary.
Three Republicans are in the race for the GOP nomination: Sen. Joe Pennacchio, a Morris County dentist; college professor Murray Sabrin; and Andy Unanue, a member of the family that founded Goya Foods.
