I don't think an economic shock would hurt Clinton because Trump has defined himself as erratic, impulsive, and having bad judgment. That trumps him being in the "out" party. People seeking stability will go to Clinton.
For similar reasons, I don't think a massive terrorist attack favors him, plus Clinton has shown she can present as tough and hawkish.
I am skeptical any devastating personal scandal is possible for Hillary after 25+ years in the spotlight paired with a personal scandal magnet.
1). The current President is a Democrat who has the exact political views that Hillary Clinton does. If there is a major terrorist attack or financial crisis, then Clinton will take the blame, regardless of Trump's supposedly being "erratic, impulsive, and having bad judgment." History has proved this point several times (e.g. Herbert Hoover, Jimmy Carter, George W. Bush, George H.W. Bush). Never has there been an example in which the incumbent party didn't take the blame for a major crisis because the opposing candidate was so bad.
2). That she currently has a low approval rating shows that she is not immune to devastating personal scandals, despite 25+ years in the spotlight. She will fall if a scandal is devastating enough.
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