Do celebrity endorsements matter? If so, who?

Winfrey with then-Sen. Barack Obama in Iowa in 2007. (Getty)
So Hank Williams Jr., Chuck Norris and Clint Eastwood and his chair are among those down on President Obama. OK.
(Actually, Williams hates … a lot.)
I guess Hollywood elites are OK … except the ones who are down on Mitt Romney and supporting the incumbent.
Ah, the celebrity endorsement. The question is not “Who cares?” but “Does any of this matter at the polls?”
Most celebs sit out the political rhetoric: Why tick off potentially half your audience?

Williams: “We’ve got a Muslim for a President who hates cowboys, hates cowgirls, hates fishing, hates farming, loves gays, and we hate him!” (Photo by Wade Payne/Invision/AP)
“Nothing doing on politics,” Babe Ruth once said, after refusing to have his photo taken with Republican candidate Herbert Hoover in 1928. (It caused a furor, since Ruth had posed for a campaign photo for Al Smith. He later had a picture taken with the candidate.)
But more than a few wade in, backing candidates and causes. The question is: Does the celebrity endorsement make a difference?
A 2010 study of young voters says the answer by and large is no. If anything, there can be a negative impact.
But a recent study said Oprah Winfrey’s endorsement of Barack Obama for 2008 may have been worth a million votes.
After a point endorsements become predictable based on political leanings. (That said, the appearance by the libertarian-leaning Republican Eastwood at the GOP convention caused a stir.) There is little “Wow” factor, other than the outrageousness of some of the rhetoric. Mention a name star’s name and odds are you know which way they will lean already.
So here is the question I’m wrestling with — and have yet to come up with a good answer:
What public figure’s endorsement could actually impact this presidential election? That person could come from any arena. I’m drawing a blank beyond the usual suspects; if that person exists I can’t think of him or her. Help.