Mattis’s criticism probably won’t directly affect public opinion; there just aren’t a lot of people who pay close enough attention to the news to hear what he said and who are also open to changing their minds. But it will still prove consequential. Although some Republicans will join Trump in denouncing Mattis, others will hesitate or be even less inclined to defend the president’s actions than they had been (and most hadn’t exactly been leaping to the microphones to support him in any event). Democrats were already going to condemn Trump, but they’ll likely do so more harshly now. And those who aren’t aligned with either party may well feel that a “neutral” stance now requires more criticism of the president, given that it’s not just protesters who are judging him harshly.
Meanwhile, the list of Donald Trump’s terrible personnel choices according to Donald Trump gets one name longer, as the president reacted predictably by bashing Mattis. That list now includes a defense secretary, a secretary of state, an attorney general, at least one national security adviser and at least two White House chiefs of staff. Again, there are very few people who are otherwise undecided about Trump who would be swayed by what an awful job — according to Trump himself — he has done appointing people for the most important positions in government. But it’s a remarkable record nonetheless.