Pres. Trump deliberately chose to use the slur itself instead of simply saying something like “I would never call Gov. DeSantis an Italian slur.” He’s done it before with other people and other words. This way he can claim “plausible deniability”, although some can see right through his tactics.
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It’s not for plausible deniability. It’s a light joke.
Pres. Trump’s campaign tactics are clear when he makes a “light joke” like that. He used them before, he’s using them now, and he will continue to use them. Ethnic slurs do not make good joke material. It’s not that they harm those they’re directed at, they are much more harmful to those who use them.