The James Carville/Stanley Greenberg operation "Democracy Corps" has analyzed the millennial vote for 2016, an area in which Libertarian Gary Johnson has great appeal, and concluded he's pulling more from would-be Trump votes among the millennials than harming Hillary.
The details, starting with the declaration that victory for the Democrats (which these guys want) relies on at least one relevant if: "if progressives are smart in dealing with the third party vote."
But: "according to a new web-survey of likely millennial voters in the eleven most competitive battleground states for Women's Voices Women Vote Action Fund. Democratic millennials have started to consolidate for Clinton, but their Republican contemporaries have not done the same for Trump. Gary Johnson's millennial vote is now a repository for most of those anti-Trump Republicans."
Among millennials, "Clinton now has a huge 25 point margin over Trump in a four-way race, comparable to Obama's 2012 performance with millennials in these same battleground states. Among white millennials, Clinton's margin is 14 points stronger!"
Trump's millennial pull is only 29 percent, so they conclude "Many Republican millennials are supporting Gary Johnson who is getting 14 percent of the vote." Of white millennials, Johnson is pulling 17 percent.
Self-identified Republicans in this poll of millennials are only 75 percent for Trump, with 19 percent for Johnson (as opposed to just 5 percent of self-identified Democrats who are for the Libertarian).
Strategically, these Dem operatives conclude that among millennials: "Gary Johnson's voters are mostly Republican anti-Trump voters, and progressives should allow Johnson to play that role."
Attacks on Johnson do not have much of an impact on his vote and the margin. In fact, this survey includes an experiment that shows third party voters shift 8 points to Clinton in a two-way ballot when they instead hear messages about corporate responsibility and a contrast with Trump on taxes and trickledown economics. Attacks on Johnson only produce a 1 point shift.
The trouble with any electoral strategy worrying over millennials is they tend not to vote much; as this report notes, "Our September battleground survey for WVWVAF found 71 percent of all likely voters gave the greatest possible score for enthusiasm for voting, but just 49 percent of millennials in this survey say they are extremely interested in voting this year." (Total voter turnout was around 57 percent in 2012, with millennial turnout about half of those eligible.)
The national pattern as of June showed Johnson's appeal across the board "taking" votes roughly equally from Trump and Clinton.