C’mon, you know the jingle: “Na-tion-wide-is-on-your-side.’’ We’ve listened to Peyton Manning hum that tune way too many times. Now it’s Rachel Platten’s turn.The “Fight Song’’ singer expands on those seven catchy notes in the latest advertising campaign for Nationwide. A new 30-second commercial shows Platten (at right) playing the piano and singing about having “an early morning dad side’’ (cut to a scene of a father attempting to braid his daughter’s hair), and a “how do I retire at 50 side’’ (a bespectacled office worker does a trust fall into the arms of his colleagues) and a “sure, you can keep the kitty side’’ (a young girl and her dad bring a white kitten home to mom). As you can expect, it ends with Platten singing the familiar phrase. And Platten isn’t the only pop star singing for Nationwide. The new advertising campaign, which launched Aug. 4, includes a commercial featuring country music star Brad Paisley playing guitar andsinging aboutthe different ways the insurance company protects its members.