Since last spring, people kept saying that the two things they feared about Trump vs. Clinton is that either the email scandal might lead to an indictment or there might be a terrorist attack that would cause people to rally around Trump. The first is no longer an issue (the email stuff is, but not an indictment), and the second has proven not be so problematic. Why?
One of the consistent findings in the polls has been that most Americans (over 60%) consider Hillary Clinton be qualified. Roughly the same percentage consider Trump to be unqualified. Sure, it is astonishing that Trump can get this far and be this close yet be widely considered to be unqualified. Indeed, how can people vote for him if they think he is unqualified? Because of many things, including party identification and all that goes with it, but in part because the Trump voters who think he is unqualified either think he will have good advisers (um, have they seen his campaign team?) or that the system will mute his impact.
But when something like an Orlando or now bombs in NYC happen, it causes people to think a bit more about the qualification problem which then offsets the "rally around the xenophobe" dynamic. In these events, people can respond to terrorism in two ways: fear the people who are somehow connected to it and choose the candidate who promises to "deal" with them or fear that Trump will screw stuff up. These probably mostly offset, so that the terrorist incidents do not shake up the race.
The race is where it is at largely because of voter identification--most Republicans will vote Republican, most Democrats will vote Democrat, most independents are not really that independent and will vote however they always lean. The fundamentals that favor Clinton will not go away. The media is starting to turn a bit on Trump thanks to his Birther press conference that was not a press conference is important since the false equivalence stuff may start to lessen (see WashPo's take on the email stuff). This might lessen his dominance of the airwaves, which has been the other reason why the race is tighter than it should be.
I am still pretty confident despite 538 lowering their probably of an HRC victory to under 60%. Why? Because I cannot imagine Trump doing well in the three debates--too undisciplined, too lazy, too ignorant, too easy to bait. Folks are betting on whether he will lash out at the moderator or curse out Clinton. He may not do those things, but he will not able to be disciplined for more than an hour x 3.
More importantly, I think the election will be determined by the usual stuff even though Trump should have been disqualified roughly 176 times. Oh, and the gap in qualifications will matter when people actually have to vote.