Weak economies overseas have reduced demand for U.S. goods and, as a result, for better-paying U.S. jobs in manufacturing. Government spending cuts have taken a toll on some middle-class jobs, too.
Many employers have also discovered that they can use technology to do tasks more cheaply and efficiently than office workers used to do. And some have found that they can shift middle-class jobs to low-wage countries such as China.
By contrast, most lower-paying jobs â¿¿ from waiters and hotel maids to store clerks, bartenders and home health care aides â¿¿ can't be automated or shipped abroad.
"You're always going to have jobs in the retail sector," says Michael Evangelist, a policy analyst with the liberal National Employment Law Project, which advocates on behalf of low-wage workers.
Consider Mike Ulrich, 30, who earned a master's degree in public administration in May from the University of Colorado. Ulrich hasn't been able to find work that requires a college degree. Instead, he works at a hardware store in Spokane, Wash., earning the state's minimum wage: $9.19 an hour.
Not all July's new jobs were low-paying. Local schools hired more than 10,000 teachers and other employees. Financial firms added 15,000.
The surge in part-time employment began in April.
Jason Furman, the new chairman of the White House's Council of Economic Advisers, says part-time employment has been inflated by the across-the-board budget cuts that began to bite in March, forcing some federal workers to take time off without pay.
Analysts say some employers are offering part-time over full-time work to sidestep the new health care law's rule that they provide medical coverage for permanent workers. (The Obama administration has delayed that provision for a year and into 2015.)
But Furman disputed the idea that the health care law will ever drive companies to favor part-timers over full-timers and says the notion makes even less sense now: "Why would they shift people to part-time for something that's not going to happen until 2015?"