McCaskill grew even less effective when Mark Halperin did something TV interviewers too rarely do: He demanded substance. Give “three specific positions” of Sanders that “are too far left,” he insisted. “I am not here to be critical of my colleague Senator Sanders,” McCaskill responded, absurdly. But Halperin caught her, noting that, “With all due respect, you already were: You said he was socialist and not electable.”

Then things got interesting. The specifics McCaskill offered were that Sanders “would like to see Medicare for all in this country, have everybody have a government-insurance policy,” that “he would like to see expansion of entitlement,” and that “he is someone who is frankly against trade.”

If Hillary actually goes after Sanders on these specifics, the Democratic race will get very interesting very fast. A debate about Obamacare versus single-payer health insurance, about expanding Social Security versus restraining its growth, and about the merits of free trade would be fascinating. But I doubt it’s a debate Hillary wants to have. She is, after all, running a campaign based on generating enthusiasm among the party’s liberal core. By taking bold, left-leaning positions on immigration, criminal justice, and campaign-finance reform, she’s trying (and so far succeeding) to erase her reputation from 2008 as a timid triangulator unwilling to offer big change. Yet the more Hillary emphasizes her opposition to single-payer health care, her opposition to expanding Social Security, and her support for free trade, the more she undermines her own strategy. By taking on Sanders on these issues, Hillary also implicitly takes on Elizabeth Warren, who has made expanding Social Security and opposing the Trans-Pacific Partnership two of her recent crusades.