Why are so many young Americans who supported Sen. Bernie Sanders for president now gravitating toward the Libertarian candidate Gary Johnson?

That's a problem that Keep America Great PAC, a pro–Hillary Clinton group, is looking to solve with a new ad targeting Sanders voters in Ohio, Pennsylvania, New Hampshire, Florida, Colorado, Virginia, North Carolina, Iowa, Michigan, and Nevada.

"If you support Bernie Sanders, you don't support Gary Johnson," the spot says, over the RJD2 track "Ghostwriter." What follows is a litany of Johnson clips rattling off libertarian ideas, intercut with clips of Sanders saying "no," shaking his head, and looking uncomfortable.

In clips throughout the ad, Johnson advocates eliminating taxes, privatizing state prisons, embracing corporate money in politics, ending student loans and Obamacare, and brushing off concerns about global warming.

The ad climaxes with a checklist comparing Johnson to Sanders on the issues, in which the Libertarian fails each category. Clinton, the ad says, passes with flying colors.

It's a message Democrats are looking to hammer home to young voters as some national polls indicate Johnson made a 13-point gain with younger voters from August to September, while Clinton still struggles to hold onto millennials' attention.

Johnson likes to emphasize he's down with the kids in opposing the drug war and foreign wars, but his free market economics aren't quite in step with a generation that's warming to socialism.

Francesca Lucia, the PAC's national finance director, said the ad is targeting swing states online but may hit TV in the near future.

"Right now we're spending heavily to promote it online because it's targeted at young people, and we're much more likely to reach them there as opposed to anywhere else," she said.