The Democratic presidential primary has apparently settled down into a four-way race. Elizabeth Warren briefly took the lead awhile ago, but has since fallen back. Joe Biden is leading once more in the high 20s, while Sanders is about tied with Warren in the high teens. Then Pete Buttigieg is bringing up the rear, having surged to about 10 percent in just the last week or so. (Everyone else is in low single digits and either flat or falling.)

Many Democratic voters are surely weighing their options. But Bernie Sanders is the strongest choice to lead the Democratic ticket in 2020. Here's why.

Let's begin by going through the candidates. Joe Biden's polling is remarkably steady and is based on his support among the Democratic rank-and-file, especially black voters in the South. But he really is an awful representative for these constituencies. He is deeply implicated in many of the policy disasters that devastated the middle class and helped give rise to President Trump: deregulation, bankruptcy reform, austerity, mass incarceration, the war on drugs, and on and on. Biden is also shockingly out of step with the times — boasting of his friendships with segregationist Dixiecrats and asserting that he can still get bipartisan compromises with a party that conspired with Trump to gin up a fake investigation of Biden's own family. Even the Obama presidency (Biden's main claim to fame) was disastrous in many ways for the middle class — especially black homeowners, who were wrecked by his administration's decision to use homeowner bailout funds as a backdoor bank bailout. But at the very least we do not need someone who thinks 1970s comity between the parties (which incidentally relied on white racist backlash) can be restored.

And while Biden polls the best against Trump in a head-to-head, he is also a fumbling campaigner who can't even raise much money, and is clearly trying to coast to the nomination. There is a real risk he would not be able to put forth maximum effort in a grueling general election.

Then there's Pete Buttigieg, who has risen up as a fresh face who seems smart, with a good balance between progressivism and realism. In reality, he is quite clearly a cynical shapeshifter — a guy who abandoned Medicare-for-all not because it is bad policy (it isn't), but because he spotted an opening in the center by making dishonest attacks against it. He also has a disturbing tendency to treat black people like campaign props, and a history of talking a big game and then capitulating to a corrupt status quo. As Ryan Grim details, he fired the first black South Bend police chief (which was later rescinded to a demotion), reportedly in part due to pressure from racist white officers who didn't like a black man in charge. The Young Turks reports that recordings show one officer said, "It is going to be a fun time when all white people are in charge." It's no wonder Buttigieg has almost no black support.

Also, Buttigieg is just preposterously inexperienced. He would be 39 on inauguration day, and thus the youngest president in American history, with nothing but the mayoralty of the 306th-largest city in the country and a single seven-month tour in Afghanistan as a low-level officer as experience. Among presidents, only Trump would have had less before taking office.

Experience isn't everything, of course (there have been some terrible presidents with lots of experience) but the federal government is a massive and complicated machine, and this time especially we need someone who can work its levers aggressively. We don't need someone who will learn on the job, much less someone who clearly just wants power for its own sake. Oh, and Buttigieg also polls the worst by far against Trump.

Then there is Elizabeth Warren, who is far better than either Biden or Buttigieg. Her relative acceptability to both Team Sanders and Team Clinton is surely why she rose so high in the primary polls. Pace Nathan Robinson, her record of fighting corporate abuses is strong, and her policy platform is also very good. She and Sanders are surely the top choices for anyone who takes the Democratic Party's supposed commitment to social justice and equality seriously.

Yet pulling off that kind of a straddle is hard, and Warren has recently shown a troubling tendency to waffle on her previous commitments, notably on Medicare-for-all, which annoyed supporters but did not convince critics — which is probably why her polling has fallen of late. Moreover, her foreign policy thinking is distinctly less bold than that of her domestic policy. She was standard-issue Israel partisan for years, only shifting left when she started prepping a presidential run — and even then, she is still less specific than Sanders about pressuring the Israeli government to end its occupation of Gaza and the West Bank. And unlike Sanders, she was distinctly hesitant to call the recent military coup in Bolivia what it clearly was. She also polls third-best against Trump, though by a small margin.

Finally, there is Bernie Sanders, who has probably the hardest core of support, yet has failed so far to break out of the pack, probably because so many loyal Democrats view him with suspicion for challenging Hillary Clinton. But this attitude is unwarranted.

To start with, his foreign policy thinking is far and away the best in the field. Since his 2016 run he has staffed up with experts critical of the imperialist "Blob," and developed a policy framework that would set the United States as the center of an alliance of democratic, egalitarian nations to contest the worldwide rise of right-wing authoritarian oligarchy.

The president has near-dictatorial foreign policy powers, and Sanders is the only one talking seriously and consistently about sharply rolling back the American empire — ending the war in Afghanistan, cutting the bloated defense budget, and halting support for various bloody proxy wars. Astoundingly, he actually got a bipartisan bill through Congress that would have halted U.S. support for the Saudi war in Yemen — incidentally demonstrating that Sanders is actually a canny legislator with a long history of negotiating policy wins where he could get them (though Trump vetoed the Yemen bill, of course).

And while Warren's platform is good, Sanders' is better where it counts. There is no question about his support for Medicare-for-all — especially not some goofy promise that he would try to pass it in two pieces. Like Warren he supports worker codetermination (in which firm employees would elect part of their corporate board) but also supports ownership funds that would reserve part of a company's stock to be controlled by workers. Most critically, his climate spending package would be more than three times the size of hers — the only policy program of any candidate that is anywhere close the to the size of the problem it is supposed to solve.

Sanders also polls second-best against Trump, just slightly behind Biden — a remarkable fact given hostile media treatment and years in the spotlight as the most left-wing major politician in the country.

And this brings me finally to theories of change. Either Sanders' or Warren's full agenda would be non-starters in the current Congress, and probably not much more likely after 2020 if current trends hold. Sanders argues that he can upturn the political landscape by activating the enormous population of disaffected non-voters, who are disproportionately non-white, young, and less educated, both to elect new representatives and senators and pressure existing ones. (And sure enough, his base of support is more rooted in these groups than any other candidate.) This is the formula he followed as mayor of Burlington, which was a smashing success, though at an obviously far smaller scale.

To be sure, this is a pretty optimistic idea. But it's also the only strategy even proposed that would change the political system fast enough to deal with the multiple crises bedeviling the United States — above all climate change. A recent United Nations report showed that the world must cut emissions by 25 percent by 2030 to stay under 2 degrees of warming, and 55 percent to stay under 1.5 degrees as agreed in the Paris Climate Accords. Meanwhile China, the largest emitter by far, is slashing its renewable subsidies and planning dozens of new coal power plants.

Without U.S. leadership and pressure, there is frankly no chance of getting even close to these goals. If Joe Manchin is the swing vote in the Senate for the next several years, the consequences could be disastrous beyond imagining. Sanders' "political revolution" might be a long shot, but we must have something like it to stave that off.

Now, of course Sanders is not perfect. He would be the oldest first-term president in American history, and suffered a heart attack earlier this year. Though he appears to have recovered nicely, there is still a non-trivial chance he would die in office. At a minimum he would need a solid and much younger running mate — someone who would be trusted and experienced enough to share presidential duties, in case they need to step in and take his place.

But alas, you never get the absolutely ideal candidate in politics, and all such choices involve some kind of risk. And on balance, Bernie Sanders is clearly the strongest candidate the Democratic Party can put up against President Trump in 2020 and beyond.