SUPER TUESDAY clarified the Democratic presidential race: The contest is now between former vice president Joe Biden and Vermont Sen. Bernie Sanders (I). Both candidates insist that their overriding goal is to beat President Trump. As the primary battle moves into a new phase, this commitment will be tested. The candidates should be judged on whether they avoid scorched-earth tactics that poison the chances of a Democratic victory in November.

There can — and should be — principled disagreement. In a Tuesday evening speech, Mr. Sanders previewed his line of attack on Mr. Biden, noting the former vice president’s vote for the Iraq War, positions on entitlement programs and support for past trade agreements. That’s fair: Mr. Biden should have to defend his support for the Iraq War, and the Democratic Party could use a vigorous debate on trade, particularly now that Mr. Trump has shown that the protectionist alternative is a dead end. Democrats should tangle over how to make entitlements such as Social Security and Medicare financially stable for another generation.

But intimating that Mr. Biden wants to take away Americans’ Social Security benefits would be a rank distortion. Likewise, it would be fair for Mr. Biden to challenge Mr. Sanders’s record on guns but not to make it seem as though Mr. Sanders is personally responsible for millions of gun deaths.

Iyanla Fuller, a sophomore at the College of Charleston, says she fears for the future of the country. (The Washington Post)

If the race were based only on policy, there would be clear distinctions between Mr. Biden and Mr. Sanders but also substantial agreement on where the country should move — in a progressive direction. But the competition could turn very ugly if the candidates question each others’ motives and intentions.

Mr. Sanders said Wednesday that he does not want a “Trump-type” campaign, but one based on “serious issues.” Yet in the same news conference, he implied that Mr. Biden would be controlled by corporate interests: “Does anyone seriously believe that a president backed by the corporate world is going to bring about the changes in this country that working families and the middle class and lower-income people desperately need?”

Earlier this week, Mr. Sanders declared that the corporate and political “establishment” was uniting to defeat him, because “they” are nervous that he would fight for higher wages, battle climate change and expand health-care coverage. That is a funny way to describe fellow candidates, all of whom share these goals, leaving the race and endorsing one of his rivals. It also disregards the millions of rank-and-file voters, many of them African American, who turned out Tuesday to vote for Mr. Biden.

Mr. Sanders’s comments suggest he could stoke his base to believe that Mr. Biden’s selection as Democratic nominee would be illegitimate. They dismiss any agenda short of his vision for radical change as tantamount to favoring the status quo.

Such rhetoric, tellingly echoed by Mr. Trump, plays right into the president’s hand.

Read more: