One of Hillary Clinton's closing arguments in the countdown to the Iowa caucuses is that she's the best bet to win a general election against Donald Trump, Sen. Ted Cruz or other GOP hopefuls pushing ideas that are anathema to Democratic voters.

A new ad released by Mrs. Clinton's campaign seeks to remind Democrats of the stakes in the 2016 race. It features a montage of belligerent-sounding Republican candidates and casts Mrs. Clinton as best positioned to keep them out of the White House.

"Who's the one candidate who can stop them? Hillary Clinton," the narrator says.

How about Bernie Sanders, Mrs. Clinton's chief rival for the Democratic nomination? Could he stop them? Or is Mrs. Clinton correct that she's the most electable of the three Democratic candidates?

Conventional wisdom suggests Mr. Sanders would have a tough time of it. He is, after all, 74 years old, and an avowed democratic socialist who in the past has staked out positions outside the guardrails of mainstream American political thinking. While running for governor of Vermont in 1976, Mr. Sanders wrote an op-end in a local newspaper that called for government ownership and "worker control" of the "major means of production."