For some time now, one of the big challenges looming over the presidential candidacy of Sen. Bernie Sanders (I-Vt.) has been the narrowness of his coalition. The Iowa results, and a variety of polls, appeared to show that Sanders’s support was heavily concentrated among young voters, those without a college degree and those who self-identify as very liberal.

Meanwhile, Sanders seemed to be struggling among older, educated, moderate and African American voters — suggesting that he had yet to broaden his coalition to the point where he might achieve breakout status.

The new Post/ABC News poll, which shows Sanders vaulting to a sizable national lead, also suggests that he might be in the process of surmounting this problem as we speak.

The Post poll finds Sanders has the support of 32 percent of Democratic voters and Democratic-leaning independents nationwide, up from 23 percent in Post polling from January. By contrast, Joe Biden is at 16 percent, a sharp plummet from 32 percent last month, and Mike Bloomberg has lifted himself to 14 percent on the force of his massive self-funded advertising expenditures, where he’s battling with Sen. Elizabeth Warren (D-Mass.) (12 percent) for third place.

But, crucially, even as Sanders has surged, he’s also broadened his support among key demographics. I’ve created this chart, comparing Sanders’s performance among these subgroups of Democratic and Dem-leaning independent voters in January with his performance among them in the new poll:

As you can see, Sanders has substantially expanded his performance among moderate/conservative Democrats, to 28 percent. He’s more than doubled among African Americans, also to 28 percent.

It’s true that Sanders is still struggling with older voters, which could still prove to be a real problem. It’s also true that his gains were far more limited among college-educated whites. But he’s still crossed the 20 percent mark among those voters, to 22 percent. Indeed, Sanders has over 20 percent among every group except older voters — including ones he’d previously struggled with.

Meanwhile, Sanders is building rather substantially on the groups he previously performed well among, rising to 50 percent among younger voters, to 35 percent among non-college whites, and to 39 percent among liberals. He’s up to 30 percent among white Democrats.

Obviously there is still a very long way to go. We will know more after the actual voting in the Nevada caucuses and the South Carolina primary, which feature more diverse electorates than the ones in Iowa and New Hampshire that were so hospitable to Sanders.

The 14 contests on Super Tuesday, which also feature diverse electorates, also loom as unpredictable for Sanders. We don’t yet know which candidates might have dropped out by then — potentially making it more possible for moderates to coalesce behind an alternative to Sanders.

And of course, there is also Bloomberg’s immense spending in the Super Tuesday contests, which could produce large and unforeseen effects.

It’s worth stressing again that nothing makes you seem like a winner the way … winning does. Sanders is plainly benefiting from his quasi-win in Iowa and his clear win in New Hampshire. Fair or not — performances in primaries are not necessarily predictive of general election prowess — perceptions of his “electability” are expanding, because he’s winning contests, and leading in polls in others, and because the media is treating him, deservedly, as the front-runner. These perceptions can change with circumstances, or at least grow more muddled.

But if you’re Sanders’s campaign advisers, you’ve got to be pleased with his expanding performances among groups that were long thought to be potentially hostile to him.

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