Despite ample evidence that Gary Johnson, the Libertarian Party presidential candidate who earned 3% of the popular vote in 2016, pulled more votes away from President Trump than Hillary Clinton, Neera Tanden still has an axe to grind with actress Susan Sarandon for supporting Jill Stein, the Green Party's also-ran.

News like this has become commonplace and every once in a while, I just want to thank @SusanSarandon for her campaigning, not just voting, third party. Instead of working to stop Trump, she helped him. https://t.co/0K23l0ZJZC — Neera Tanden (@neeratanden) August 19, 2019

Despite the fact that the entirety of Stein's vote share still wouldn't have won Clinton the White House, frantic lefties have continued to mull their nightmare scenario: a serious third-party contender emerging in 2020 to hand Trump a second-term.

I don't know what the Mooch or anyone else is up to. It would be nice to see Trump have to fight a primary against a real conservative, but don't let anyone - left or right - sell you on a 3rd party run against Trump in 2020. That only helps Trump. The math is clear. — Tom Nichols (@RadioFreeTom) August 17, 2019

Nichols isn't definitely wrong, but unless Democratic front-runner Joe Biden fails to maintain his persistent and compelling lead, it's hard to see how he's right.

In 2016, voters were asked to choose between a bloviating billionaire accused of sexual assault by multiple women and a corrupt career politician who protected her husband from credible accusations of sexual assault by multiple women. Hillary Clinton had always been a pretty conservative Democrat, and Donald Trump had been a pretty liberal Republican, after he was a Democrat. Voters trusted neither of them, resulting in a largely non-ideological choice between two liars with similar politics and similarly malevolent personal conduct.

The coming race could present voters with a very different kind of choice. If Biden does clinch the nomination, the revolt against Republicans in the suburbs of 2018 could recur. Biden's politics aren't too different from those of Hillary, but on a personal level, most Americans like him and trust him. Even if a certain coalition of disaffected Republicans only marginally prefers Biden over Trump, it's hard to imagine those in Ohio risking a Trump win by voting for a hypothetical third party candidate instead of Biden himself.

No compelling candidate has declared a third-party bid as of yet, but the calculus is simple, regardless of who runs: A third-party bid hurts whichever major party runs too extreme. Trump's actual policies may have proven far more mainstream and successful than voters imagined in 2016, but his rhetoric and Twitter musings have painted his presidency as distinctly partisan and polarizing.

A contrast with Biden would emphasize that flaw on Trump's part. The other Democratic front-runners would contrast with Trump chiefly through their political extremism.

Asking a small business owner in the suburbs of Orlando to trade new tax cuts for a return to Obama-era policies with an affable and known quantity back in the White House is reasonable. Telling them to trade Trump's Twitter meltdowns for the abolition of the private health insurance market and wealth taxes or estate taxes to fund free college is quite another ask. Trump is loathed enough by Democrats that any nominee will get the vote of the Left, but if the nominee is too extreme, that crucial never-Trump vote will go to a third party candidate instead.

Unless a candidate with already sky-high name recognition like Michael Bloomberg or Michelle Obama or one with a clearly defined constituency like Justin Amash runs for a third-party bid, it's unlikely that any who entered the race would gain much traction anyway. The ephemeral "Libertarian moment" of 2016 proved a bust, and it's unclear who's willing to wade into the waters until the Democratic primary is decided. But in any case, Democrats ought to remember that a third-party challenger only threatens their odds if they elect the unelectable.