I never understood this logic. How does it help to shift the discussion to something even worse?
Trump winning in 2016 has caused even people to hate him to think he is some master strategist when he would be fairly popular and a shoe in for reelection if he would stop saying stupid s*** every 5 seconds or walking into a new scandal every couple days.
It is a horrible tweet, but it is just a tweet. Remember when he tweeted that transgenders were no longer welcome in the military?
Trump isn't a master strategist, but he's better than people give him credit for. The people who support him don't care about his tweets, they know they are specifically meant to do things like this. Trump is nuts, but he's not that crazy. The military wouldn't even carry out such an attack.
You can't even find "impeachment" anywhere on CNN today. Pelosi is already pushing legislation to limit Trump's powers. So, we're already validating it by "debating" it.
Condone it publicly, but do everything in the frame of impeachment. "We think this kind of threat is detrimental to the country, especially with a President who has been impeached". "Impeached Presidents shouldn't be making threats over twitter". You can react to it without letting it steer the narrative.
I hate to tell you too, but Joe Six Pack doesn't care about bombing Iran, and they certainly don't care about a tweet that suggests the possibility.