Sanders gains strength – and mainstream Democrats worry

Could Bernie Sanders win the Democratic nomination and then get routed in the general election because he’s seen as too liberal? That concern lurks. Then again, 2016 blew up the political playbook.

With just 12 days until the Iowa caucuses and a dominant front-runner yet to emerge, what once struck many as an unlikely outcome – “Democratic nominee Bernie Sanders” – is suddenly looking like more of a possibility.

Mr. Sanders’ polling strength is eliciting anxiety within certain quarters of the Democratic Party not unlike the reaction many Republicans...