LOS ANGELES — Phil Spector’s lawyers face an “uphill battle” in the closing stages of the music producer’s trial for murder, according to legal experts, amid doubts about the appearance of a key defense witness.

The defense is expected to present the rest of its witnesses over the next few weeks before the jury retires to decide if the 67-year-old Mr. Spector is guilty of killing Lana Clarkson, a B-movie actress who died in a dawn shooting at his home in 2003.

So far, Mr. Spector’s lawyers have introduced witnesses in an attempt to show that Clarkson, who worked as a nightclub hostess, was depressed at the time and likely died by her own hand.

But legal commentators following the trial are unconvinced by the case presented by the defense team, headed by Bruce Cutler.

“They have been trying to make the case she [Clarkson] was suicidal, and I think it’s been uneven,” Laurie Levenson of Loyola Law School in Los Angeles said. “Certainly, she had financial difficulties, and she was off and on depressed, but I’m not sure it quite makes the case yet that she went over to a total stranger’s house, borrowed his gun, and killed herself.”