General political science literature says that when there is some national crisis, like a terrorist attack, we observe what we call a "rally 'round the flag effect," in which the president's approval rating goes up. However, as the work of Richard Brody has shown, this is not a mechanical effect. It is conditional on "elite consensus," meaning that cue-givers around the country, from across the political spectrum, have to send signals to the populace that we should unify around the president in a time of crisis. The complication, of course, is that Trump's hyperbolic and frequently violent rhetoric against Democrats, the press, and in general, anyone he doesn't like, along with Republicans' propensity to spread insane conspiracy theories about people like George Soros (remember that insanity that he was paying the protesters during the Kavanaugh hearings?) make a rally effect highly unlikely. Democrats rallying around Trump after a bomb kills a Democrat amid Trump's rhetoric? Not likely.
Could there be a reverse effect? Quite possibly. We haven't seen one, empirically. We have seen rallies collapse after consensus collapses, but not a reverse rally, precisely. Yes, presidential approval drops when the economy drops, but that's different from the type of crisis that produces a rally, which usually involves some threat to the country. Then again, we haven't seen a president whose rhetoric is as violent as Trump's, so we don't know what happens if that violent rhetoric is accompanied by an actual, successful terrorist attack.
As I said, though, we don't know what happens because we've never had a president who calls for violence and praises violence as often as Trump. Uncharted waters, as we have been sailing for years. We don't know what happens, but don't assume that Americans get horrified into decency. People have been predicting that reaction to Trump's vileness for years, and the reaction keeps not happening. There are just a lot of sick motherfuckers out there who love Trump's violent shit. Don't think that the bomber represents more than one person. It probably doesn't. And in all likelihood, the bombs won't work, because mailing pipe bombs is not exactly a mastermind plot. This is Richard Reid-level terrorism, not Osama Bin Laden-level terrorism. The cheering crowds when Trump yells his love of violence? They represent a lot of people.
And they vote.