Gut-wrenching gyrations in financial markets early in the year helped summon the specter of a new recession. Now, warning signs are coming mostly from the U.S. economy itself.

Hiring is slowing, auto sales are slipping and business investment is dropping. America’s factories remain weak and corporate profits are under pressure. All are classic signs of an economic downturn, and forecasters have certainly noticed. In a Wall Street Journal survey this month, economists pegged the probability of a recession starting within the...