(CNN) Sen. Bernie Sanders may win the Democratic Iowa caucuses a week from today. But he lost the coveted Des Moines Register endorsement to fellow liberal Sen. Elizabeth Warren over the weekend. And, in explaining why they opted for Warren over Sanders, the Register's editorial board hits on a key question that the Vermont democratic socialist will need to answer in the coming weeks and months.

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"While Elizabeth Warren and Sanders are in lockstep on many positions, concerns about Warren's potential for divisiveness are magnified with Sanders. As a self-identified democratic socialist, someone who has set himself apart from the Democratic Party during his congressional career, let alone breaking bread with Republicans, could he build the consensus needed to govern?"

(Sidebar: The Register released the strengths and weaknesses of each of the candidates it didn't endorse online. It's here .)

In two sentences, the Register's editorial board perfectly encapsulates the doubts (and worries) that lots of Democratic voters -- not ardently behind Sanders already -- have about him.

Forget whether or not he can beat President Donald Trump. If Sanders is elected president, can he actually bring the country or its politicians together to solve problems?

Because, make no mistake, that's what Democratic voters want. In a CNN national poll released earlier this month, almost 6 in 10 (58%) Democrats and Democratic-leaning voters said they would rather a presidential nominee "advocate for policies that have a good chance of becoming law, even if the changes they make aren't as big," while just 36% said they would opt for a nominee who would "advocate for policies that would result in big changes, even if they have a lower chance of becoming law."

Many establishment Democrats -- led by Hillary Clinton -- have long criticized Sanders for the fact that he was not a "real" Democrat -- having long affiliated as an independent, only changing his affiliation to Democrat to run for president in 2016 and now 2020.

"I was running against somebody who publicly advocated President Obama being primaried. Right? So it was difficult to have what I considered to be a fair-minded debate about, 'Okay we have a successful, two-term president, where do we go from here?' with somebody who wasn't a Democrat, who criticized both President Obama and me."

And in Clinton's more recent critique of Sanders -- which went public last week -- she hit on the exact concerns about Sanders that the Register editorial board noted.

"He was in Congress for years," Clinton said in a Hulu documentary entitled "Hillary." "He had one senator support him. Nobody likes him. Nobody wants to work with him. He got nothing done."

Is Clinton technically correct? Well, it's hard to say whether anyone in Congress liked (or likes) Sanders. We can do a little bit better with Clinton's claim that Sanders "got nothing done." According to GovTrack , Sanders has been the primary sponsor of seven pieces of legislation that became law since being elected to the House in 1990. (He won a Senate seat in 2006). Two of those bills were to rename post offices while a third was to designate March 4, 1991 as "Vermont Bicentennial Day."

But as any close watcher of politics and Congress in particular will tell you, judging success in Congress by sponsored bills that become law is a decidedly imperfect measure of relevance or success. The problem? We don't have any significantly better metrics.

Here's the most curious thing about all of this: Sanders' hardcore supporters like the idea that he isn't a going-along-to-get-along guy. They like that he isn't friends with a bunch of Washington establishment types -- the sort of people who have failed to deliver progressive solutions for decades. And he won't be that sort of president either. Which is a good thing, not a bad thing in their eyes.

That view, of course, is not the majority view in the country or in the Democratic Party. Which doesn't mean that those views can't change. But what's clear -- from the DMR piece as well as the Clinton critique -- is that Sanders still has work to do to change minds on that front.