Gary Johnson is slightly good for Hillary

As a never-Trumpist and a Gary Johnson supporter, I'm interested in questions like whether Gary Johnson helps or hurts Hillary Clinton. If you assume that Johnson > Hillary > Trump and that Johnson hurts Trump on net, then promoting Johnson seems beneficial (with an option to vote for Hillary if the election is close in your state based on immediate pre-election polls), whereas if Johnson hurts Hillary on net, then at least there's more of a tension to be resolved.

Now if you follow the national polls, you often find that the four-way polls (including Johnson and Stein) cut into Hillary's lead slightly. For example, as of right now, the RCP average of two-way polls gives you Clinton +0.9 nationally, while the average of four-way polls gives you Clinton +0.7 nationally. Based on that, people have been saying that Johnson helps Trump, which is troubling for never-Trump Johnson supporters.

But of course the election isn't a national poll, it's an aggregation of state polls. In Georgia, where I live, the two-way poll average gives you Trump +4.0 while the three-way poll average (maybe Stein isn't on the ballot here?) gives you Trump +3.7. So at least in Georgia, Johnson helps Clinton, which is good. On the other hand, there are different results in different states: Trump's lead in Ohio actually increases when you go from two-way polls to four-way polls. So that suggests you need to do a state-by-state analysis.

Also, people casually report this effect as whether Johnson helps or hurts Hillary, whereas in most states we're talking about the aggregate effect of Johnson plus Stein. Plausibly, Stein (as a Green, like Nader) takes most of her support from people who would otherwise vote for Hillary, whereas Johnson's support is more mixed because some libertarian policy ideas sound traditionally conservative and others sound traditionally liberal. So if Johnson-plus-Stein in the aggregate hurts Hillary, maybe most of that hurt comes from Stein, not Johnson. So you need to correct for the Stein effect when calculating the Johnson effect.

Based on all that, friend and sometime co-blogger Jacob Levy has run some math and determined that Johnson slightly helps Hillary. You should read the whole thing, but here's the core:

In short: the Johnson-Weld campaign has emphasized all along that it draws from left and right alike. They're right. Indeed, they're so right that it does not seem like the Johnson-Weld campaign- even at high levels of support, like the 13% they had in Quinnipac, or the 10%+ they had in 42 states in WaPo/Survey Monkey- will tilt the presidential election one way or the other. (Remember, even at those levels of support there doesn't seem to be even a single state that they're flipping.) But the balance tilts a little bit toward "they're taking votes from Trump," for an overall result that's something like a 2% net benefit to Clinton. If the race stays as close as it now looks- which I don't expect- that could matter.

If I can do this math, so can the Johnson-Weld campaign- and so can the Clinton campaign. Stein is a real problem for Clinton. But the Johnson-Weld ticket is an opportunity. If I'm right that Trump's support among Republicans remains fragile- if traditional Republicans are uneasy keeping company with neo-Nazis and the Klan, or worried on national security grounds about Trump's ignorance and instability, or constitutionalist enough to dislike his embrace of authoritarians abroad and of strongman executive demagoguery at home- then increasing visibility for Johnson-Weld could well have a very asymmetrical effect.

Clinton supporters when confronted with this are prone to say things like "those Republicans should vote for Clinton." And we're starting to see the signs of some Clinton surrogates gearing up for a scorched-earth campaign against Johnson on those grounds. But those disgruntled Republicans won't vote for her- for reasons having to do with decades of annoyance with the Clintons, or for reasons of the basic tribalism of partisan identity. Taking half a step away to vote for former Republicans is a lot psychologically easier for many Republicans than taking a full step away to vote for a Clinton. And so, if anything, I would expect the net benefit Johnson-Weld provide to Clinton to grow a bit if his support grows further. In any case, on current evidence, the Libertarian ticket is having a largely neutral but slightly pro-Clinton effect on the race as a whole, and this fact is being widely misreported because people are improperly lumping together the effects of Johnson and Stein, then attributing that effect to the Libertarians along in a classic fallacy of division.

Again, read the whole thing.