(Stephen Chernin/AFP/Getty Images)

What do this election cycle’s most watched presidential advertisements have in common?

Of course, many of them feature grainy black and white images of the other guy and ominous voiceovers. Plenty of factory workers in hard hats. And there’s the “...and I approved this message” line that we’ve all heard a few times too many.

But there’s another common theme among the most-aired advertisements: the Clintons.

Of the 20 top ads (10 from each campaign), a full four of them feature a Clinton. Two of GOP candidate Mitt Romney’s ads show Hillary Clinton, while two of President Obama’s star Bill.

(Big hat tip to our pal the Fact Checker, who identified with the help of Kantar Media, the 10 ads from each campaign with the greatest spending.)

Obama employs the former president in two ads, “Clear Choice” (“we need to keep going with his plan,” Clinton intones) and “Forward” (in which the former prez slams Republicans: “they want to go back to the same old policies that got us in trouble in the first place.”)

Romney, for his part, used footage of Hillary Clinton from her failed campaign against Obama for the 2008 Democratic presidential nomination. Two of his ads, “Shame on You” and “No Evidence“ use a clip from the 2008 primary race in which Clinton wags her finger at Obama for a recent attack. “Shame on you, Barack Obama,” she chides.

The appeal of both is obvious. Bill Clinton is a powerful surrogate for Obama, a reminder of a Democrat presiding over a flush economy — and he’s popular and thought to play well in swing states. And that footage of Hillary Clinton chastising Obama is effective because it’s criticism from a fellow Democrat — and it exploits the old rivalry between the two former competitors.

So there might not be a Clinton on the ticket (perhaps we’ll have to wait for 2016 for that), but they’re still all over presidential politics.