The decision of a Hollywood mogul, Steven Spielberg, to endorse Senator Clinton’s presidential campaign is leading some Democratic operatives to conclude that Senator Obama’s star is losing some of its luster.

Mr. Obama’s entry onto the national political stage earlier this year dazzled many in the entertainment elite, but his unremarkable performances in recent debates and a lack of attention-grabbing policy proposals has some prospective supporters returning to Mrs. Clinton’s fold.

“This ‘dated Obama, married Mama’ phenomenon, similar to what afflicted Howard Dean, is happening at a rapidly accelerating pace,” a California-based Democratic political adviser, Daniel Newman, said. “Obama is to some extent a victim of his own early success — when you come blazing out of the gate, it makes it very difficult to beat expectations.”

In his statement yesterday, Mr. Spielberg was careful not to deride Mr. Obama or any of the other contenders. “I’ve taken the time to familiarize myself with the impressive field of Democratic candidates and am convinced that Hillary Clinton is the most qualified candidate to lead us from her first day in the White House,” Mr. Spielberg said. “Hillary is a strong leader and is respected the world over. As president, she will bring America back together, rebuild our prestige abroad, and ensure our protection here at home.”

Mr. Spielberg gave donations to Mrs. Clinton, Mr. Obama, and a former senator from North Carolina, John Edwards. The deal with Mrs. Clinton was apparently sealed when Mr. Spielberg and Mrs. Clinton spent some time together on May 30 at a fund-raiser he co-hosted in Santa Monica. “That sort of completed the process for him,” a political adviser to Mr. Spielberg, Andrew Spahn, said.

Two of Mr. Spielberg’s partners at Dreamworks SKG, David Geffen and Jeffrey Katzenberg, are firmly in Mr. Obama’s camp. In February, Mr. Geffen sparked a row between the Democratic presidential contenders when he hosted a high-profile fund-raiser for the Illinois senator and gave an interview sharply questioning Mrs. Clinton’s electability and President Clinton’s ethics.

Mr. Spahn said the flap over the comments by Mr. Geffen, who was once a supporter of the Clintons, did not play a role in the timing of Mr. Spielberg’s decision to back the former first lady.

A film and television producer in Mr. Obama’s camp, Lynda Obst, said the political newcomer still has a strong base of support in Hollywood. “Support for Obama doesn’t really seem to be shifting. It was early, and I think it’s deep, and I think it’s young,” she said. “It’s more emotional and political than it is calculated. It’s not based on people figuring out necessarily where their bread is buttered and who’s going to sleep in the Lincoln Bedroom.”

Ms. Obst said a large crowd turned out Tuesday afternoon to hear Mr. Obama’s wife, Michelle, speak at a fund-raiser at the Los Angeles home of a television journalist and “Dancing with the Stars” contestant, Giselle Fernandez.

A Democratic strategist who advises clients in the entertainment business, Christopher Lehane, said Mr. Obama’s momentum in California has been arrested in part by Mrs. Clinton’s success at winning over the political establishment, including the mayor of Los Angeles, Antonio Villaraigosa. “When you have folks of that stature, it certainly impacts the entertainment community,” the adviser said.

Mr. Lehane said people open to Mr. Obama’s candidacy are waiting to see some substance or some flash. “They really haven’t seen a second act,” the strategist said. “By definition, that is causing people to take a second look at everyone else.”

One active Democrat with close ties to prominent Hollywood figures said her mind is still open to Mr. Obama but that Mrs. Clinton seems like a safer choice. “If someone put a gun to my head and I had to choose today and the person had to start today, it would be Hillary,” the Democratic adviser, who asked not to be named, said.

Asked about some Democrats who are yearning to see more from Mr. Obama, Ms. Obst said, “I think that’s fair. … I think all Democrats have the right to get to know all their candidates.”

Ms. Obst, whose productions include films such as “One Fine Day” and “How to Lose a Guy in 10 Days,” said she is politically smitten with Mr. Obama. “I haven’t felt this way in many, many, many, many electoral cycles,” she said. “He is dazzling but doesn’t try to dazzle.”

The producer said Mr. Obama’s deliberate approach is just right for a campaign that has gotten an unexpectedly early start. “He’s going to pace himself,” she said.