Taiwanese director Ang Lee's aching story of unfulfilled love rides into the ceremony armed with a leading eight nominations and is tipped to win awards including best picture and best director, in a year when Oscar snubbed big studio blockbusters in favour of low-budget dramas tackling weighty topics.
The 78th annual Academy Awards will start at 0130 GMT on Monday in front of an audience of glamorous movie stars who will first strut their stuff for the cameras in the world's most famous red carpet fashion show.
An official of the Academy of Motion Pictures Arts and Sciences said: "We're almost there, we've only got the last frenzied hours to go."
About 3300 VIP guests were due to arrive at the foot of the massive red carpet to be greeted by a blaze of flashbulbs and the screams of 300 hand-picked fans.
As the stars primped and preened, hundreds of workers put the final touches on preparations for the ceremony while an army of police and security personnel sealed off the area around Hollywood's Kodak Theatre to ward off any potential terror threat.
But inside the Kodak Theatre, Brokeback is facing a different sort of threat in the form of a serious last-minute challenge from Crash, Paul Haggis's gripping low-budget story of racial prejudice.
Awards expert Tom O'Neil said: "The buzz is that the threat to Brokeback from Crash is now very real."
"The buzz is that the threat to Brokeback from Crash is now very real."
Tom O'Neil, awards expert
He added that Brokeback, Capote and Transamerica may have led to "gay fatigue" among Oscar voters.
O'Neil and several other pundits, however, remain confident that Brokeback will carry off best picture and up to four other Oscars.
Hollywood Reporter online columnist, Marty Grove said: "I'm still thinking Brokeback is on the fast track to take best picture after winning all the other top Hollywood awards, but I think Crash is the alternative film."
Brokeback's rising star, Australian Heath Ledger, is up for best actor, while co-stars Jake Gyllenhaal and Michelle Williams are up for best supporting performer awards.
Favoured to win
The film faces off against the six-times nominated Crash, about a group of ethnically diverse people whose lives collide in a Los Angeles car accident, and George Clooney's political drama Good Night, and Good Luck, which is also up for six Oscars.
Small budget movies look set to
win big this year
Munich - Steven Spielberg's epic about the aftermath of the Palestinian massacre of Israeli athletes at the 1972 Olympics - and Capote, about gay US author Truman Capote, both of which are up for five Oscars, round off the best picture contenders.
Capote star Philip Seymour Hoffman is widely favoured to win best actor Oscar for his staggering title role in Bennett Miller's biopic, facing off against Ledger and Joaquin Phoenix, who played country star Johnny Cash in the biopic Walk the Line, nominated for five Oscars.
Also competing in the category are David Strathairn, for his role as newsman Ed Murrow in Good Night, and Terrence Howard for Hustle and Flow.
The firm favourite for best actress is Walk the Line singing co-star Reese Witherspoon, with Felicity Huffman next in line for her powerful turn as a transsexual in Transamerica.
Other contenders are Britain's Keira Knightley, 20, who could become the youngest best actress laureate if she wins for Pride and Prejudice, and previous Oscar winners Judi Dench, nominated for Mrs Henderson Presents, and South African Charlize Theron, who played a female miner in North Country.
Briton Rachel Weisz is widely tipped to win best supporting actress for her role as an activist fighting the pharmaceutical industry in Kenya in The Constant Gardener, with Williams - Heath Ledger's real-life fiancee - also a serious contender for her portrayal of his cuckolded wife in Brokeback.
Frances McDormand is also nominated as best supporting actress for North Country, as is Amy Adams for Junebug and Catherine Keener for her role as writer Harper Lee in Capote.
Heartthrob Clooney is tipped to beat out Gyllenhaal and win best supporting actor for his role as a CIA spy in the oil industry thriller Syriana.
Clooney, who is up for three Oscars, also faces competition from Paul Giammati for Cinderella Man, Matt Dillon for Crash, and William Hurt for A History of Violence.
Facing off against frontrunner Ang Lee for best director are Capote's Miller, Haggis for Crash, Clooney as director of Good Night, and Spielberg for Munich, the only big-budget film in the major categories.
The politically outspoken Clooney is also nominated for his best original screenplay for Good Night, the story of newsman Ed Murrow's crusade against the repression of the US and-Communist witch hunt of the 1950s.