"Brokeback" has four chances to win at Sunday's show, which airs live at 8 p.m. EST on TNT and TBS.
Heath Ledger earned a best-actor nomination for his role in "Brokeback" as a family man concealing a homosexual affair with an old sheepherding buddy, played by Jake Gyllenhaal, who received a supporting-actor nomination. Michelle Williams, who plays Ledger's wife, was among supporting-actress nominees. "Brokeback" was also nominated for best performance by its entire cast.
The film biography "Capote" and the ensemble drama "Crash" follow "Brokeback" with three nominations each, among them honors for their entire casts and for lead actor Philip Seymour Hoffman ("Capote") as Truman Capote and supporting actors Don Cheadle and Matt Dillon ("Crash").
Along with "Brokeback," "Capote," "Crash," the Edward R. Murrow tale "Good Night, and Good Luck" and "Hustle & Flow," the story of a pimp and drug dealer forging a career as a rap singer were all nominated for best performance by the entire cast.
Joining Ledger and Hoffman in the lead-actor category were Russell Crowe as Depression-era boxer Jim Braddock in "Cinderella Man"; Joaquin Phoenix as singer Johnny Cash in "Walk the Line"; and David Strathairn as newsman Murrow in "Good Night, and Good Luck."
Lead-actress nominees were Judi Dench as a society dame who starts a nude stage revue in 1930s London in "Mrs. Henderson Presents"; Felicity Huffman in a gender-bending role as a man preparing for sex-change surgery in "Transamerica"; Charlize Theron as a woman leading a sexual-harassment lawsuit at a mining company in "North Country"; Reese Witherspoon as Cash's soul mate and eventual wife, June Carter, in "Walk the Line"; and Ziyi Zhang as a poor girl who becomes a belle of Japan in "Memoirs of a Geisha."
Huffman also was nominated for best actress in a TV comedy series for "Desperate Housewives," a role that earned her an Emmy last year.
"Desperate Housewives" co-stars Teri Hatcher, Marcia Cross and Eva Longoria, who along with Huffman took four of the five TV musical or comedy nominations at the Golden Globes (none of them won), were all shut out for guild nominations.
The show's entire cast, however, was honored with a nomination for comedy ensemble, along with "Arrested Development," "Boston Legal," "Curb Your Enthusiasm," "Everybody Loves Raymond" and "My Name Is Earl."
Nominated for TV drama ensemble were "The Closer," "Grey's Anatomy," "Lost," "Six Feet Under" and "The West Wing."
Along with Cheadle, Dillon and Gyllenhaal, the other contenders for best supporting-actor were George Clooney as an undercover CIA agent in the oil-industry thriller "Syriana" and Paul Giamatti as boxer Braddock's manager in "Cinderella Man."
Joining Williams as supporting-actress nominees were Amy Adams as a Southern waif in the comic drama "Junebug"; Catherine Keener as Capote pal Harper Lee, author of "To Kill a Mockingbird," in "Capote"; Frances McDormand as an ailing miner in "North Country"; and Rachel Weisz as a slain humanitarian-aid worker in "The Constant Gardener."
Poised between the Golden Globes and the Oscars, the SAG Awards, which air live from Los Angeles' Shrine Exposition Center, boast that is the only show of its kind where winners are selected purely by actors' peers.
Thirteen awards (five film categories and eight TV) will be bestowed, as well as a Life Achievement Award to Shirley Temple Black.
SAG nominations are chosen by 4,200 randomly chosen members of the union. The guild's full membership of 98,000 is eligible to vote for winners.
The Associated Press contributed to this report.