Steinberg's support is a major coup for fellow Democrat Dickinson, who is likely to face Assemblyman Richard Pan, D-Sacramento, in the 2014 Senate race. Steinberg will be termed out.
Dickinson and Pan were slated for an intraparty showdown in the 7th Assembly District in 2012 before Pan moved to the Pocket area to run in the 9th district.
Dickinson was elected to the Sacramento County Board of Supervisors in 1994 and served for 17 years before his Assembly run in 2010. In addition to Steinberg, Dickinson will announce a lengthy list of endorsements, including US Sen. Dianne Feinstein, Sacramento County supervisors Phil Serna and Don Nottoli, Sacramento City Councilmembers Bonnie Pannell and Steve Cohn and former Sacramento Mayor Heather Fargo.
Pan has a campaign committee open and is fundraising, but will likely not announce his intention for the Senate seat while the Legislature is in session, said Doug Herman, Pan's campaign strategist.
Herman said Steinberg's endorsement of Dickinson is not a deal breaker.
"There will be a lot of support for Pan and Dickinson," said Herman, who was Assembly Speaker John A. Perez's campaign consultant. "In the end, it will come down to who they are. Endorsements won't make a difference in this race."
Editor's note: This post was updated with Herman's comments at 4:44 p.m. on June 18, 2013.
PHOTO: Assemblyman Roger Dickinson, D-Sacramento, in a 2010 file photo. The Sacramento Bee/Hector Amezcua