Republican presidential candidate George H.W. Bush, left, and Democratic candidate Michael Dukakis shake hands before their final debate in Los Angeles in October 1988. (Lennox McLendon/Associated Press)

Susan Estrich, national campaign manager for Michael Dukakis’s 1988 presidential campaign, is the Robert Kingsley professor of law and political science at the University of Southern California.

It has been 30 years since an ad featuring the mug shot of “Willie” Horton, the infamous murderer who raped a woman while out from prison on furlough, became the theme of the 1988 presidential election. It was paid for by a committee that you’ve never heard of. And it blamed Democratic candidate Michael Dukakis, for whom I served as campaign manager, for letting him out.

Of course it tested well. People were predisposed to be afraid of a black man they see in a mug shot. And then you hit it home and blame it on your opponent.

Today, President Trump is doing the exact same thing. Except in this case, he subbed in a Mexican immigrant for a black person and tweeted it out himself. There’s a lesson from the Horton ad episode that Democrats should take to heart: Don’t let him get away with it.

We knew the Horton issue was coming long before it hit the national airwaves. I knew the story far better than Lee Atwater (the mastermind of the ad who later apologized to me) ever could. And I know what it cost, not only in percentage points but also in the way it changed for the worse the way we have dealt with race and crime. It poisoned a difficult issue, divided us just when we needed to be united. And it won votes.

Dukakis has always been a decent and honorable man. But he hated stunts. So as Massachusetts governor, when the pro-death-penalty crowd brought along a woman to town, press in tow, who had been raped by a murderer named William Horton while he was out on furlough, he wouldn’t meet with her.

We knew during his presidential campaign that this would eventually become an issue nationally. You didn’t get a more powerful symbol of racial fear and hatred than a black man raping a white woman. And so we came up with a plan to take it on.

We had the ad team coming up with ads, the speechwriters inserting whole paragraphs into his nomination acceptance speech about his family’s own losses to violent crime, cameras filming the sites where his brother was killed and his father mugged and left for dead. Then-Arkansas Gov. Bill Clinton crafted the lines to use for the Horton question at the debate; when Dukakis didn’t use it, the floor fell out completely. We were loaded with dirt on George H.W. Bush, including the rumored adultery that I famously fired Donna Brazile, then-deputy field director for the Dukakis campaign, for mentioning to the media. Dukakis would have none of it.

It wasn’t just that he believed the furlough program improved discipline, although he did believe that. He didn’t think Americans would respond well to a short Greek immigrant’s kid from Harvard calling people names, even if that’s precisely what his opponents were doing to him. He kept talking about a “strong positive” campaign, and he actually meant it.

It took Lloyd Bentsen, the much-respected moderate Texan who was running as Dukakis’s vice president, to denounce the Horton ad as racist. Most of the criticism for the ad came after the election. But by then, the damage had been done.

For the next decade and more, every conversation I had about sentencing policy, or the unfairness of the drug laws, or the craziness of all the mandatory sentencing laws financed by the prison guards union, or the need to deal straightforwardly with the underlying roots, everyone from the president to the local council member would remind me of Horton. “You, of all people . . .”

The conventional lesson was that being “soft on crime” could kill you, however bad the racial overtones. I’m not sure it was ever really true, but it certainly isn’t now. The United States has changed, even though our president has not. The right lesson from the past, the one to remember, is the danger of not standing up to racism when you see it and not fighting hate when it faces you.

Call it what it is. The president’s new ad is racism.