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.

To continue to read Karl Rove's column in the Wall Street Journal, click here.

Karl Rove joined Fox News Channel as a political contributor in February 2008. He also currently serves as a columnist for the Wall Street Journal. Mr. Rove helped organize the political-action committee American Crossroads. His latest book is "The Triumph of William McKinley: Why the Election of 1896 Still Matters" (Simon & Schuster, 2015). Follow him on Twitter @KarlRove.