Seeking to turn a liability into a strength, the latest ad from Attorney General hopeful Chris Koster frames his switch from Republican to Democrat as a tough decision based on principles, not politics.

That is, of course, a stark contrast to the contention of party rival Jeff Harris, who has repeatedly questioned whether Koster’s conversion is more a marriage of convenience than true devotion. 

Either way, Koster’s new commercial carries over the slogan from his last spot — “All Prosecutor. No Politics.” — providing an easy segue to address his party crossover.

“When the Republican Party turned its back on stem cell research, workers’ rights, child daycare,” the ad says, “Koster turned his back on the Republican Party. It wasn’t easy, but leadership never is.”

The ad also highlights Koster’s pursuit as Cass County prosecutor of serial killer John Edward Robinson, who lured victims over the Internet using the handle “Slave Master.”