I'd be an obvious act of war. Republicans and most average Americans would feel justified in joining with Russia to rip Ukraine in half in retaliation. Even Europe would be hesitant to defend Ukraine because we had all just seen the Ukrainian president (fatally?) assault the king of america on live television.

However, within the country I think if any national level democratic politician was serious about the threat posed by trump they should use their political position to get access to him and then tackle him and start punching. Obviously they'd go jail. Obviously there'd be a national scandal. But the dude is old a hard fall would kill him. In the long run, it'd do way more good than holding up a "this isn't normal" index card.

He’d immediately be shot probably

Knowing what fragile snowflakes trump and the republicans are they’d probably have him jailed or try to have him shot and just hand Ukraine to Putin on some sort of shared exploitation agreement.