There are no hard and fast rules for pick­ing a polit­ic­al an­them. Jon Bon Jovi gave Chris Christie per­mis­sion to use his mu­sic on the cam­paign trail. Bernie Sanders enters and exits massive ral­lies to the tune of Neil Young’s “Rockin’ in the Free World.” Mar­tin O’Mal­ley some­times plays gui­tar on the trail. (The O’Mal­ley cam­paign has also taken mu­sic­al ad­vice from a Mary­land ra­dio sta­tion DJ.) Rick San­tor­um has an of­fi­cial cam­paign theme song titled “Take Back Amer­ica,” and you’ll of­ten hear “Homegrown” by the Zac Brown Band at Jeb Bush events.

“You have to have a soundtrack. If you don’t have a soundtrack, you don’t have as clear an iden­tity with voters. It’s something to be very de­lib­er­ate about. If you’re not choos­ing wisely, you’re miss­ing an op­por­tun­ity,” said An­drew Bar­low, a speech writ­ing con­sult­ant for Re­pub­lic­an can­did­ates, in­clud­ing Ted Cruz, and founder and CEO of me­dia firm Over­flow Com­mu­nic­a­tions.

A lot of thought goes in­to se­lect­ing a song. At a rally, cam­paigns look for mu­sic that can wake up a crowd. For ad­vert­ising and pro­mo­tion­al videos, mu­sic that in­spires or in­stills fear, de­pend­ing on what kind of mes­sage the cam­paign wants to send, may work best. And it’s not just the ven­ue. Geo­graph­ic loc­a­tion mat­ters as well. Pres­id­en­tial con­tenders some­times spot­light loc­al fa­vor­ites as a way of telling the audi­ence: “I’m just like you!”

“These are not de­cisions you ar­rive at lightly. You want something con­sist­ent with your val­ues, you want it to be en­er­get­ic, and some­times there’s a deep­er mean­ing,” said Rick Ab­bruzzese, a former O’Mal­ley aide and con­sult­ant for a pro-O’Mal­ley su­per PAC.

At least one 2016 con­tender is out­sourcing the task. Hil­lary Clin­ton’s cam­paign paid $9,000 to en­list the aid of a boutique mu­sic agency based in Port­land, Ore­gon, re­cords from the Fed­er­al Elec­tion Com­mis­sion show. Bill­board re­por­ted that the agency “has been tasked with mu­sic su­per­vi­sion and cre­at­ive sup­port” for a series of Clin­ton cam­paign videos, in­clud­ing the video that launched Clin­ton’s 2016 bid. (A spokes­per­son for the mu­sic agency said the firm is “ex­cited about our re­la­tion­ship with the Hil­lary cam­paign,” but ad­ded that an in­ter­view would not be pos­sible due to a nondis­clos­ure agree­ment. The Clin­ton cam­paign de­clined to com­ment.)

But pres­id­en­tial can­did­ates don’t al­ways get the mu­sic they want. Car­son’s cam­paign hoped to use the gos­pel rendi­tion of Em­inem’s “Lose Your­self” as a 2016 an­them. But Watts, the cam­paign com­mu­nic­a­tions dir­ect­or, dis­covered he “could not get through the pub­lish­er without empty­ing our bank ac­count.” So he settled for second-best: The cam­paign got the gos­pel choir to re­cord a song modeled after the ori­gin­al, a mu­sic­al se­lec­tion that the cam­paign has since used in a cam­paign video.