A Democrat who’s pro-weed can win, indeed. Photo: Caroline Brehman/CQ-Roll Call, Inc via Getty Imag

Donald Trump is never going to win a popularity contest. But he might just win a two-term presidency.

On the day of his election, Trump boasted a 38.6 percent approval rating — the lowest of any major party nominee since the advent of opinion polling. A wide variety of factors enabled Trump to win the White House while alienating a supermajority of the American public. The Electoral College, the exceptionally low approval rating of his Democratic rival, and the relatively low turnout rates of low-income voters were among the major ones.

But the unusually strong performance of third-party candidates was a significant minor factor. Gary Johnson and Jill Stein did not cost Hillary Clinton the election. But by collectively capturing a disproportionately left-leaning 4.3 percent of the vote (roughly 3 percent more than they’d won in 2012), they made Trump’s task considerably easier.

In 2020, Trump is all-but certain to be an unusually unpopular incumbent president. And his campaign has given every signal that it intends to run a base-centric reelection campaign, wagering that a narrow — but intense — band of support will be sufficient for eking out another Electoral College majority. The better third party candidates perform in 2020, the more viable that strategy will be: Anything that reduces the percentage of the vote a major-party nominee needs to secure pluralities in Wisconsin, Pennsylvania, and Michigan is likely to be good news for the man whose approval rating hasn’t cracked 44 percent since the third month of his presidency.

Thus, one critical task for the Democratic nominee in 2020 will be minimizing the third-party vote. But there’s little consensus about how that can be done. After all, Johnson and Stein voters are a motley crew with a diverse array of policy preferences and political attitudes. Is there really any one policy idea that could mellow the contrarianism of libertarians and Greens, alike?

If you can’t think of any, I want what you’re smoking. It doesn’t take a political scientist to see that a Democratic nominee could plausibly make inroads with (disproportionately young and 420-friendly) third-party voters by endorsing the legalization or marijuana. Many bloggers have been saying as much since the fall of 2016.

That said, hard evidence that a “soft on drugs” message could reduce third-party vote-share has been tough to score. Fortunately, Data For Progress (DFP) has finally got the hook up. In a new survey, the progressive think-tank tested the potency of marijuana legalization as a general election appeal. To do this, it presented one set of respondents with a hypothetical three-way general election between Donald Trump, a generic Democrat, and independent candidate Tulsi Gabbard — and another (demographically identical) set of respondents with the exact same race, only this time attributing a pro-legalization position to the Democrat. (Gabbard has disavowed any interest in running as a third-party candidate, but some observers find her denials unconvincing.) Here’s the exact wording of the latter question: “If the 2020 Presidential election were being held today, and the candidates were a Democrat who supports using presidential authority to legalize marijuana and Republican Donald Trump, who believes marijuana should be illegal in all cases, for whom would you vote?”

Without the information about each major-party candidate’s position on marijuana, the generic Democrat led Trump 48 to 44 percent, with 4 percent going to Gabbard. With that information, the generic Democrat’s lead over Trump widened to 50 to 41 percent, with Gabbard taking a little under 2 percent of the vote.

Although the difference here is not massive, it is large enough to be statistically significant, and potentially electorally decisive: Some prominent election forecasters believe Trump can win an Electoral College majority while losing the popular vote by 4 percentage points, but none think he could do so while losing the popular vote by 8.

Even if marijuana legalization weren’t a tonic for third-party voting, it would remain a political no-brainer. Recent polling from Pew and Gallup finds more than two-thirds of Americans support the policy. As of this writing, every major Democratic presidential candidate has made peace with legal weed — except for the one most likely to win the party’s nomination.