For the whole of 2017, Bernie Sanders has reliably measured as the most popular politician in America, by a considerable margin. So far he has used that newfound influence to fight President Trump's agenda, promote left-wing ideas like Medicare for all, and campaign on behalf of left-wing candidates for office. He's also founded a political organization and a think tank.

Will he be satisfied by this? Speculation is growing that he might run for president again, as Matt Yglesias argues. If the last couple years of political history has taught me anything, it's that attempting to handicap elections is a mug's game. But one thing is clear: Whatever he is going to do, Sanders needs a political protégé or two.

Why would Sanders run for president? He is better positioned than any other Democrat; he's made tremendous inroads with the black community, whose support for Hillary Clinton doomed his primary campaign (indeed, he now has greater approval among African-Americans than he does among any other race); and he won't face a party establishment that is already locked up behind someone else. Trump is and continues to be the most unpopular president in the history of polling; one can see why Sanders might be tempted to give it another shot.

On the other hand, it must be admitted that Sanders is quite old. He is 75, which according to the Social Security actuarial tables gives him a 3.6 percent chance of dying this year, with a life expectancy of 11.1 years. If he were to take power in 2021, he would be 79, with a 5.3 percent chance of dying and a life expectancy of 8.8 years. He seems in excellent health, but the fact of the matter is there is a sizable chance he will not make it to 2028. It's a grim thought but it simply can't be avoided.

Second, there is the fact that he really is as yet the only nationally well-known leftist in the country. Keith Ellison is getting there, and could potentially leap to the front rank of politics as Sanders has — but it hasn't happed yet. Nina Turner, often mentioned as an up-and-comer in the Sanders camp, is excellent but much less well-known than even Ellison. There are at most only a couple others — and none in Sanders' league as far as national profile. It is dangerous for a political movement to rely so heavily on a single person.

Finally, there is the political witch hunt in Vermont being conducted against Sanders by Brady Toensing, who ran Trump's Vermont campaign. If Politico's reporting is to be believed, Sanders' wife Jane rather epically botched the management of a tiny Burlington college, which has since closed down. The alleged connection to Bernie, however, is weak, and the complaint which sparked the investigation based on hearsay. But Toensing is still trying to get Trump to nominate him as U.S. attorney for Vermont, so he can try to take Sanders down with that or any other politically motivated prosecution he can whip up.

That Trump himself is the most corrupt president in history by an order of magnitude only makes it more likely that such a thing will happen. It's a grim statement about this moment in history that it's wise to assume the apparatus of violent state coercion will be used for clumsy political hatchet jobs, but that doesn't make it less plausible. (Though one should also note it's only the apotheosis of a long Republican trend.)

All this means that Sanders really ought to start using his immense popularity and influence to start grooming a successor or three. Who it is is less important than there be somebody who can help spread his message and step into his shoes, should that become necessary. If he runs for president, this will become an absolute necessity — he would not want to pick some compromise unity candidate like Lincoln did in 1864, which led to catastrophe.

I imagine this will be pretty challenging for the Vermont senator. Sanders has been a loner for most of his political career, and is known to be somewhat prickly. That is part of why he is so popular now, as only an extremely stubborn person would resist the tide of neoliberalism back in the '80s and '90s as he did. But now the tides have shifted — and actually implementing a policy vision is a different task than keeping that vision alive through a generation of darkness. It means, at the least, a substantial bench of leaders. Sanders can start by boosting up a few of the next generation of American leftists.