"He opposed the wars they oppose, just as I do — the same wars Hillary Clinton supported and sent us into," says Johnson. "Senator Sanders is one of the few politicians who tells the truth about the war on drugs. … Hillary? She is against legalization. The poll numbers are just not high enough for her, I guess."

The ad came as Alternative PAC, a pro-Johnson group founded by former tea party leader and Rand Paul supporter Matt Kibbe, announced an $848,410 haul in the past three months, on the strength of an online ad that featured an Abraham Lincoln impersonator telling voters to rebel against both parties.

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But any benefit to Johnson has been well concealed. In an average of polls collected by RealClearPolitics, Johnson's overall support has fallen to around 6.6 percent, from a high of 9.2 percent one month ago. Among millennials — as he says, the bloc he was doing the best with — some polling has shown Johnson's support falling by half. That's after the campaign Johnson obliquely refers to in the ad, an effort by Tom Steyer's NextGen Climate and the pro-Clinton Our Principles PAC to warn millennials that Johnson was a climate-change skeptic (he is not) and that a Donald Trump presidency would lead to major reversals on abortion rights.

Sanders, who has been stumping for Clinton and for down-ballot candidates with a warning against casting "protest votes," took one swing at Johnson this week. On Twitter, the Vermont senator pointed his 3.6 million followers to an investigation in the Huffington Post, looking at how the Libertarian Johnson favored private prisons when he was governor of New Mexico.