I wondered if they were originally going to have it be beer or caffeinated soda but changed their minds. They went with a sound effect from opening a beer/soda can when he cracked open at least one of the milk cartons.
I really liked that skit. It didn't strike me as mean-spirited at all, just a playful ribbing. Except maybe throwing Karl Rove off the balcony, I suppose, but eh.
ETA: My wife wonders if it's meant as a swan song for Jason Sudekis. I thought I'd heard he was going to leave after the last season, along with Kristen Wig and Andy Samberg, and we were wondering if he stuck around specifically because they needed a Romney. And now they don't need a Romney.