I believe it's a reference to this:
“Before beginning the drive, Mitt Romney put Seamus, the family’s hulking Irish setter, in a dog carrier and attached it to the station wagon’s roof rack. He’d built a windshield for the carrier, to make the ride more comfortable for the dog.
“Then Romney put his boys on notice: He would be making predetermined stops for gas, and that was it.
“The ride was largely what you’d expect with five brothers, ages 13 and under, packed into a wagon they called the ‘white whale.’
“As the oldest son, Tagg Romney commandeered the way-back of the wagon, keeping his eyes fixed out the rear window, where he glimpsed the first sign of trouble. ‘Dad!’ he yelled. “Gross!” A brown liquid was dripping down the back window, payback from an Irish setter who’d been riding on the roof in the wind for hours.
“As the rest of the boys joined in the howls of disgust, Romney coolly pulled off the highway and into a service station. There, he borrowed a hose, washed down Seamus and the car, then hopped back onto the highway."
Cliffs: Romney put his dog on his car roof while he drove for hours and it ended up crapping on his car.