Ben Domenech tweets that an appearance by GOP former first ladies makes the most sense. I agree, with the caveat that Barbara and Laura Bush aren’t big enough draws to justify drumming up suspense for a “mystery speaker” in advance. If they’re going this route to soften Romney’s image by using popular, well-known Republican women to sing his praises, they’re going to want Nancy Reagan there for star power and to bestow RR’s benediction symbolically on him. Whether she’s up to it physically I have no idea, but she was well enough last month to help launch a new exhibit at the Reagan library. We’ll see.

If not her, who? Domenech’s right that Romney’s unlikely to gamble on a red-meat conservative on Thursday night, especially one like Limbaugh or Palin whom he can’t control. He’s looking for centrist votes, to the point where he’s begun touting his RomneyCare record(!) as evidence of how good he’s been for women’s health. And if you believe Frank Luntz and Steven Law of American Crossroads, the most effective way to pull centrist swing voters away from Obama is to show them other disappointed Obama voters talking publicly about changing their minds. That’s really what Romney needs on Thursday — a 2008 Obama fan who’ll come out and make the more-in-sorrow-than-in-anger case for switching. Artur Davis would have been perfect except that (a) he’s already scheduled to speak and (b) he’s not sufficiently famous to wow undecideds with his shift. There are, however, three guys who might be.

One: Ace wonders if it’s Douglas Wilder. Remember that strange episode two weeks ago when he was spotted at a Romney/Ryan fundraiser, then told the press he’d gone to the wrong hotel while looking for an event on education? He’d spent the previous two days on Fox News dumping on Biden for his “chains” comment. Having America’s first black governor endorse a Republican over America’s first black president would be high drama worthy of “mystery speaker” suspense, but Ace hears from Bryan Preston at PJ Media that it’s unlikely.

Two: Colin Powell. No crossover endorsement was bigger for Obama in 2008 so having him cut bait on The One at the convention would be as strong a signal as you could send to voters that it’s okay to admit your mistake and atone. Powell said as recently as June that he was still undecided between Romney and Obama; a month before that, he called Romney a “good man” whom he’s known for years before going on to praise some of O’s first-term “accomplishments.” Endorsing Romney on national TV would make him instantly relevant again. Two problems, though. First, there’s enough bad blood between him and the base over 2008 that I doubt he’d want to risk being booed at the convention. Maybe he figures that the crowd will let him slide since he’s there to help them win now? I don’t know. Seems risky. Second, unendorsing a guy at the other party’s convention is a move reserved for those who either feel deep disappointment over the candidate’s performance or stand to gain something significant politically from doing so. Powell fits neither of those categories. He’s not running for anything and he still admires Obama enough to have complimented him on Iraq and Afghanistan(!) just this past weekend. There’s no reason for him to show up and torpedo the guy on Thursday, unless there’s some sort of grudge or hard feelings that we don’t know about.

Three: Hmmmmm.

A sitting Democratic senator giving thumbs down to a sitting Democratic president warrants “mystery speaker” hype, and it’s true that Manchin’s pointedly refused to endorse Obama thus far. One question, though: What does he gain from doing this? As of May, he’s leading his Republican opponent by more than 50 points. He doesn’t need to make a presidential endorsement to win; on the contrary, doing this will only make life harder for him within the Democratic caucus. I suppose he could be considering a party switch next year — which, if so, would make Todd Akin’s Missouri fumble suddenly less damaging — and is trying to ingratiate himself with the GOP now in hopes of some plum committee assignments from McConnell next year. If it’s not Nancy Reagan, I think I’d bet on him.

Oh, incidentally, Romney’s advisors say they don’t know who the mystery speaker is. Mmm hmmm.

Update: I totally forgot this one, but Jamie Weinstein didn’t. Good call:

4.) Clint Eastwood

The 82-year old actor known for his toughness and grit has already endorsed Romney. While many Hollywood actors are prone to scorn for being out of touch elitists, Eastwood doesn’t carry that image. He isn’t the Hollywood-type most Americans would recoil for injecting himself into politics. An endorsement from Eastwood, who served as mayor of a California city and who George H.W. Bush reportedly considered for his vice presidential nominee, would seem to fit the bill as someone who would work as a high-profile mystery prime time speaker.

Not a case of someone switching his vote from 2008 (Eastwood endorsed McCain), but if the name of the game is getting swing voters’ attention, you can do worse than a guy who’s been a cultural icon for 50 years.

Update: Nope, it won’t be Manchin.