Don’t do it. Do not go down that road again.

Criticize the president for his policies, his phraseology, his war strategies and terrorism-fighting approach. But don’t revive that ugly nonsense about Barack Obama not loving America, not being American, or being a closet Muslim who’s out to undermine the government from within. That nasty, fear-stoking strategy used by political opponents during Obama’s first presidential campaign has moved alarmingly from anonymous emails and bogus websites to the mouths of Republican standard-bearers.

Donald Trump does it by questioning Obama’s birth certificate. Now former New York City Mayor Rudy Giuliani questions Obama’s love for his country. “I do not believe that the president loves America,” Politico quoted Giuliani as saying at a private dinner for fellow Republican, Wisconsin Gov. Scott Walker, who seems to be gearing up for a White House run. “He wasn’t brought up the way you were brought up and I was brought up through love of this country.”

Walker has not repudiated Giuliani’s line of attack, noting only that he, himself, loves America. Shame on him. And shame on Giuliani.

Shame also on Louisiana Gov. Bobby Jindal, who as an Indian-American should know better than to play into ethnic divisiveness. “If you are looking for someone to condemn the mayor, look elsewhere,” Jindal, also a prospective presidential candidate, told Time magazine.

Giuliani’s comments, and their tacit acceptance by likely presidential candidates, reflect two disturbing trends. Even as America is on its way to becoming a predominantly brown-skinned nation, race-baiting and foreigner-bashing by politicians are growing. One key way is through the ongoing inference that Obama’s not one of us because he’s black, has a mixed ethnic background and a Muslim middle name.

As disturbing as the racial subtexts is the revival of another tactic first seen by most of us during the Vietnam War. That is to question the patriotism of anyone who suggests that, as the world’s most powerful nation, we try diplomacy and respect for other nations and religions. What provoked Giuliani was that Obama, in a speech about preventing attacks from groups like al-Qaida and Islamic State, took pains not to use the terms “Islamic terrorism” or “Islamic extremism.”

Maybe the president is being overly cautious in his effort not to antagonize most Muslims. But it was hardly worthy of attacks like Republican Sen. Ted Cruz of Texas, another likely presidential candidate, calling Obama “an apologist for radical Islamic terrorists.” Let’s not forget that President George W. Bush took pains to stress we were not at war with Islam, and his patriotism was never questioned for it.

At the start of Bush’s Iraq and Afghanistan wars, opponents got smeared as unpatriotic. Yet even though those wars and drone strikes are now Obama’s, and draw criticism from people like me, the right still declares Obama unpatriotic.

Their insistence on stigmatizing him as “other” is an ugly tactic that will leave scars on the national psyche that could take a long time to heal. If candidates won’t renounce that, if they can’t win on real issues, they shouldn’t run.