US Senate candidate Jason Kander was consistently trailing incumbent Roy Blunt in the Missouri race until this month. Many attribute the shift to a simple and memorable ad in response to criticism of Kander's position on gun background checks.
Bonus video: this ad by Missouri gubernatorial candidate Eric Greitens probably plays well with core supporters, but is pretty ham-fisted in comparison to Kander's.
Fun fact: 3% of Americans own about half of America's guns, "between 8 and 140 guns apiece, 17 on average."
• Background Checks (YouTube / Missourians for Kander) via NPR