"This thing can really catch fire," Crane said in a short interview. "If Gary is in the debates against two people who are very unpopular, it could be quite a dramatic change in how voters look at the election."

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In one of the spots, Johnson is contrasted with Hillary Clinton (seen dissembling about her email server) and Donald Trump (seen deriding a disabled reporter) as the "honorable choice" for president.

The second spot focuses more on Johnson's words, clipping sections of his spring 2016 speech to the Conservative Political Action Conference.

Both spots echo what Purple PAC did in its most successful work to date — its 2013 spots promoting Robert Sarvis, the Libertarian candidate for governor of Virginia, who held on to more of his vote than pollsters expected in a race with two fairly unpopular major party candidates. That race shares some staff, and some thinking, with the Johnson campaign, which sees 2016 as a similar shot for a breakthrough.

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But to achieve that, Johnson needs to crack 15 percent support — something no third-party candidate has done since 1992, and something made trickier by the splintering of the anti-Trump vote. In places such as New Hampshire, Johnson lacks some of the support that Ron Paul earned. The reason, say activists and writers, is that Johnson and running mate Bill Weld — the former governor of Massachusetts — present themselves as centrist alternatives to Trump and Clinton, not doctrinaire Libertarians.

In recent days, Johnson has tested that by declining to rule out a carbon tax as a replacement for some other taxes. He was pushed on that Thursday afternoon, in a live-streamed interview with the Union Leader newspaper.

"I have not proposed a carbon tax or a carbon fee," Johnson said. "In theory, it sounds great, and that's what I wanted to acknowledge. I'm not sure how it can actually get implemented. Boy, it is very complex."

"I'm allergic to the T-word," Weld said. "But they put this system in place in British Columbia, where it's revenue neutral. Then the question becomes, is it really a tax?"