For oth­er per­spec­tives on this debate, read Peter Frase and Sean McEl­wee.

In 2016, the Demo­c­ra­t­ic Party’s pre­sump­tive leader said sin­gle pay­er could ​“nev­er ever come to pass.” Today, one-third of Democ­rats in the Sen­ate, includ­ing most Demo­c­ra­t­ic 2020 hope­fuls, have signed on to Sen. Bernie Sanders’ Medicare for All bill, and more than 60 per­cent of House Democ­rats have co-spon­sored Rep. John Cony­ers’ (D‑Mich.) version.

Much of the party’s shift to the left can be attrib­uted to Sanders’ 2016 pres­i­den­tial cam­paign and his dogged pro­gres­sive advo­ca­cy. His lead­er­ship on issues such as a $15 min­i­mum wage, free pub­lic high­er edu­ca­tion and labor rights makes him the clear­est, most con­sis­tent and, yes, the most pop­u­lar voice on the polit­i­cal Left. Sanders doesn’t fol­low con­sen­sus: He makes it. Vot­ers know the difference.

There’s a wide­spread lack of con­fi­dence in our gov­ern­ment and its lead­ers, con­tribut­ing to an enthu­si­asm for out­sider can­di­dates. Insid­ers can claim expe­ri­ence, but are lim­it­ed by long records that force them to ratio­nal­ize their ​“evo­lu­tion” on sub­jects from gay mar­riage to free trade.

Out­siders are vul­ner­a­ble to accu­sa­tions that they’re knownoth­ings, but with­out pri­or com­mit­ments, they’re able to make bold promis­es — be they about low health­care pre­mi­ums or a bor­der wall — while claim­ing inde­pen­dence from polit­i­cal debts.

What makes Sanders unique is that he can claim the best of both iden­ti­ties. He’s a bat­tletest­ed, non­par­ti­san ide­o­logue with a record as long as it is reflec­tive of polit­i­cal integri­ty. From gay equal­i­ty to the Iraq War, Sanders’ abil­i­ty to get it right so often isn’t just luck — it’s indica­tive of his com­mit­ment to prin­ci­ples over par­ty, pol­i­tics or donors. It’s that com­mit­ment that won him over­whelm­ing sup­port in work­ing-class (and I don’t mean white work­ing-class) strong­holds in the Midwest.

But despite all the qual­i­ties in his favor, there are three cred­i­ble con­cerns pro­gres­sives have raised about Sanders.

Some doubt the via­bil­i­ty of a can­di­date who iden­ti­fies as a demo­c­ra­t­ic social­ist, even if they sup­port those pol­i­tics. This under­es­ti­mates socialism’s pop­u­lar­i­ty. Sanders him­self earned 43 per­cent of the 2016 pri­ma­ry vote despite low name recog­ni­tion, a cor­po­rate media black­out (they wouldn’t even air his cam­paign announce­ment) and oppo­si­tion from the Demo­c­ra­t­ic Nation­al Com­mit­tee. Unlike Hillary Clin­ton, Sanders has con­sis­tent­ly main­tained a dou­ble-dig­it lead over Trump in polls.

Can­di­dates like Alexan­dria Oca­sio-Cortez are show­ing that ​“demo­c­ra­t­ic social­ism” is just as like­ly to pro­voke a red wave of pro­gres­sive can­di­dates as a Red Scare. The Left must resist self-defeat­ing skep­ti­cism; as Nathan Robin­son put it in Cur­rent Affairs, ​“The most seri­ous bar­ri­er to accom­plish­ing polit­i­cal goals is the peo­ple who insist that they can­not be done.”

A more per­sua­sive argu­ment is that, at age 79 in Novem­ber 2020, Sanders would be the old­est pres­i­dent-elect in his­to­ry — eclips­ing Ronald Rea­gan, who start­ed his sec­ond term at 73 and end­ed it rumored to be suf­fer­ing from dementia.

This con­cern is not irra­tional. But both Joe Biden and Trump, if elect­ed, would also be old­er than Rea­gan. Con­cerns about age should be root­ed in empir­i­cal mea­sures of health, not ageism.

After spend­ing this spring report­ing on Sanders, hus­tling to keep up as he bounced from Nashville to Jack­son, Miss., I have few doubts about his phys­i­cal sta­mi­na. And unlike in Reagan’s case, there’s no sign of any decline in his men­tal fac­ul­ties. In the worst case, vice pres­i­dents — espe­cial­ly well cho­sen, ide­o­log­i­cal­ly well­matched vice pres­i­dents — pro­vide a safeguard.

But the issue most like­ly to define the pri­maries is not age, but race.

In 2020, Sanders will lit­er­al­ly pale in com­par­i­son to like­ly estab­lish­ment hope­fuls Kamala Har­ris and Cory Book­er, and frus­trate vot­ers who are tired of an end­less stream of white male presidents.

In some ways, that’s how it should be. Diverse can­di­dates mat­ter in part because they are like­ly to bring diverse per­spec­tives. But diver­si­ty means very lit­tle if it doesn’t trans­late into poli­cies that speak to the inter­ests of women and peo­ple of col­or — inter­ests that I believe are best advanced by pro­gres­sive candidates.

Eliz­a­beth War­ren, the most like­ly pro­gres­sive alter­na­tive to Sanders, has much to rec­om­mend her, includ­ing a pow­er­ful record of advo­ca­cy against the very cor­po­rate inter­ests that rile Sanders fans. But it’s wor­ri­some that she’s failed to shake Trump’s racist taunts of ​“Poc­a­hon­tas” by giv­ing a sat­is­fac­to­ry expla­na­tion of her self-iden­ti­fi­ca­tion. After all, she’ll face much worse in 2020.

Democ­rats must advance the can­di­date who best rep­re­sents the inter­ests of the Amer­i­can peo­ple — par­tic­u­lar­ly the most vul­ner­a­ble — and who can best com­mu­ni­cate those inter­ests to vot­ers. It’s what cit­i­zens deserve in a (vague­ly) rep­re­sen­ta­tive democ­ra­cy. More cyn­i­cal­ly, it’s also the path toward elec­toral victory.