Some economists, however, say the surge in reliance on part-time workers will fade as the economy strengthens and businesses gain more certainty over how they will be impacted by Obamacare.
Executives at several staffing firms told Reuters that the law, which requires employers with 50 or more full-time workers to provide healthcare coverage or incur penalties, was a frequently cited factor in requests for part-time workers. A decision to delay the mandate until 2015 has not made much of a difference in hiring decisions, they added.
"Us and other people are hiring part-time because we don't know what the costs are going to be to hire full-time," said Steven Raz, founder of Cornerstone Search Group, a staffing firm in Parsippany, N.J. "We are being cautious."
Raz said his company started seeing a rise in part-time positions in late 2012 and the trend gathered steam early this year. He estimates his firm has seen an increase of between 10 percent and 15 percent compared with last year.
Other staffing firms have also noted a shift.
"They have put some of the full-time positions on hold and are hiring part-time employees so they won't have to pay out the benefits," said Client Staffing Solutions' Darin Hovendick. "There is so much uncertainty. It's really tough to design a budget when you don't know the final cost involved."
The delay in the Obamacare employer mandate "confused people even further," said Bill Peppler, managing partner at Kavaliro, a technology staffing firm in Orlando, Fla. "When we talk to customers, I still don't think anyone has a handle on this."
Obamacare appears to be having the most impact on hiring decisions by small- and medium-sized businesses. Although small businesses account for a smaller share of the jobs in the economy, they are an important source of new employment.
Some businesses are holding their headcount below 50 and others are cutting back the work week to under 30 hours to avoid providing health insurance for employees, according to the staffing and payroll executives.
(Read more: So-so summer: Job growth disappoints; rate drops)
Under Obamacare, any employee working 30 hours or more is considered full-time. An effort to trim hours might have helped push the average work week down to a six-month low in July.
"As organizations and companies reduce the hours of part-time workers, they still have to replace the capacity, so they go out and hire additional part-time workers," said Philip Noftsinger, president of CBIZ Payroll in Roanoke, Virginia, which manages payroll for more than 5,000 small businesses.
Some large companies are also leaning more heavily on part-timers.
Wal-Mart Stores has been hiring more part-time workers, although it says the move is to ensure proper staffing when stores are busiest and is not an effort to cut costs.
Spokesman Kory Lundberg said the world's largest retailer promotes about 75,000 people from part-time to full-time work each year and is on track to do so again in 2013.
Similarly, a memo that leaked out from teen and young adult retailer Forever 21 last week showed it was reducing a number of full-time staff to positions where they will work no more than 29.5 hours a week, just under the Obamacare threshold.
In a statement, the company said the move will affect fewer than 1 percent of its U.S. store employees, and was taken to better align staffing with sales expectations—not to lower costs under the Affordable Care Act.
Some public school boards and local governments, including the city of Long Beach in California, are also cutting hours.
"The difference between 30 and 40 hours can be the difference between being able to make ends meet month-to-month," said Heidi Shierholz, a senior economist at the Economic Policy Institute in Washington.
"That contributes to reduced living standards for American families and translates into having less income to spend on goods and services, which holds back the economy."