President Obama came into office promising to unify America, but he has made political discourse meaner and more cynical. Whenever Mr. Obama is playing a weak hand, he questions the motivations of those who disagree with him and mangles the truth to undermine any criticism.

Take the president’s recent statement that Republican opponents of his nuclear deal with Iran are making “common cause” with hard-liners in Tehran who chant “Death to America.” Or consider his warnings that those opposed to the deal are “big check writers to political campaigns” and “the same array of forces that got us into the Iraq war”—a charge that borders on thinly veiled anti-Semitism.

It is not only Mr. Obama’s tone that has polluted America’s political dialogue; it is also his blatant disregard for the facts. “If you like your health care plan, you can keep it” may be his most famous lie, but it is far from his only one.

Hillary Clinton has foolishly begun mimicking Mr. Obama’s practice of impugning the motives of political opponents, as her lead in the polls shrinks and perceptions that she is untrustworthy and out of touch with ordinary people grow.

“Republicans are systematically trying to stop millions of American citizens from voting,” she tweeted last week, a reference to GOP efforts to pass state voter ID laws. These laws, which require people to present photo identification at their polling places, are clearly designed to protect the integrity of the ballot. Democrats claim that these laws suppress minority voters, but a 2012 analysis by Reuters showed that after Georgia’s voter ID law went into effect in 2006, black turnout increased four percentage points in 2008 compared with 2004, and seven percentage points in 2010 compared with 2006.