Wisconsin Governor Scott Walker (R-WI) participates in a panel discussion at the American Action Forum in Washington, in this January 30, 2015 photo. (Yuri Gripas/Reuters)

A NOTABLE moment of the 2008 presidential campaign came when Republican nominee John McCain faced down at a rally some of the more vitriolic haters of Barack Obama, who were shouting that the Democrat was a “terrorist” and an “Arab.”

“No, ma’am,” Mr. McCain said. “He’s a decent family man [and] citizen that I just happen to have disagreements with on fundamental issues, and that’s what this campaign’s all about . . .

“We want to fight, and I will fight,” the candidate continued. “But I will be respectful. I admire Senator Obama and his accomplishments, and I will respect him.”

You might hope that such lessons in character wouldn’t need to be relearned, but if so you would be disappointed. At a private dinner Wednesday for Wisconsin Gov. Scott Walker, a likely candidate for the 2016 GOP presidential nomination, former New York mayor Rudy Giuliani said: “I do not believe that the president loves America. He doesn’t love you. And he doesn’t love me. He wasn’t brought up the way you were brought up and I was brought up, through love of this country.”

Mr. Giuliani has dived off the deep end before, so there was nothing all that surprising in his latest poison. But as Post columnist Dana Milbank points out, his remarks presented an early test of character for Mr. Walker, and Mr. Walker failed spectacularly. At the dinner, the governor said nothing. Even the next morning, having had a night to ponder, Mr. Walker chose not to lift himself out of the Giuliani sewer. “The mayor can speak for himself. I’m not going to comment on what the president thinks or not. He can speak for himself as well,” Mr. Walker said on CNBC. “I’ll tell you, I love America, and I think there are plenty of people — Democrat, Republican, independent, everyone in between — who love this country.”

Mr. Walker likes to present himself as a man of courage, based on his record in Wisconsin, but maybe facing down public-sector unions doesn’t tell you all that much about the bravery of a Republican governor. On two occasions in recent days, he has proved himself incapable of saying basic truths that might offend some of his potential voters: First, that evolution is real, and second, that an honorable politician criticizes his opponent’s policies, not his patriotism. Sen. Marco Rubio (R-Fla.) showed his understanding of that principle Friday when he said, “I believe the president loves America. His ideas are bad.”

Any reader of this page knows that we find plenty to criticize in Mr. Obama’s foreign policy. But the questions about his patriotism that emerge from a venomous stew of racism, xenophobia and echo-chamber cable television chatter are ludicrous and say a great deal more about the critics than about the object of their criticism. It was particularly pathetic to see Louisiana Gov. Bobby Jindal (R), another would-be president, scrambling to steal some of the spotlight of Mr. Walker’s cowardice by issuing a statement titled, “Gov. Jindal Refuses to Condemn Mayor Giuliani.” The only response to that can be, who cares?

We have entered what will be a long and often acrimonious campaign season. Let’s hope voters will find at least some candidates in both parties who can meet the McCain standard of common decency and common sense.