The campaign has been conducted on the premise of uninterrupted prosperity. But the economy could just as easily disappoint. The new president would then strain to pay for campaign commitments. He would look bad compared with Bill Clinton, and prolonged economic underperformance could permanently damage his reputation and his party's prospects. Remember Herbert Hoover.
Memory then becomes a powerful psychological tailwind.
People remember bad times and better appreciate the good. Going the other way is a political typhoon. A mediocre politician can easily succeed when times are improving. Even the most skilled may fail when they are not. Prevailing economic wisdom supports the campaign's exceptional optimism.