A new Gallup Poll shows that 81 percent of adults say that Oprah’s endorsement of Barack Obama will have little impact on whether they will back his candidacy.

Some 8% say they are more likely to vote for him based on the endorsement and some 10% say they will vote against him because of it. Only 16% of adults said that endorsements in general were important to them, while 38% say they are not important at all.

The poll also found that Winfrey’s favorability rating was at 66%, but that is down from the levels in the mid-70s from previous surveys.

It’s not a total wash. Write Gallup’s Joseph Carroll and Jeffrey M. Jones:

“Winfrey’s endorsement of Obama will likely aid his candidacy only at the margins, and that would not be enough for him to overcome his current large deficit to Clinton. But Winfrey’s appeal to certain groups – such as women — could help Obama perform better among these groups than he would otherwise.

“Among groups of voters that view both Obama and Winfrey favorably, such as blacks, the endorsement may not change minds but may primarily reinforce existing views. That reinforcement could help motivate voters to turn out to vote or to work in support of Obama’s candidacy.”

One caveat: How many respondents will really admit that they are moved — or even influenced — to vote for someone based on an endorsement?

Update: Meanwhile, the Obama campaign is trying to diffuse the impact of another endorsement, that of Donnie McClurkin. The Chicago Sun-Times’ Lynn Sweet reports that a gay minister will open the gospel performances that are part of campaign events this weekend.