The healthcare law's Republican critics have complained loudly about its effects on small businesses and the unintended consequence of employers cutting their part-time workers' hours.
But there had been no real effort on Capitol Hill to actually solve that problem, in part because many conservative Republicans object to any effort to "fix" the healthcare law, hoping it collapses on its own.
Collins said Wednesday she has heard significant interest in changing the employer mandate but acknowledged that many in her own party have resisted efforts to improve the law.
"I personally do not think that is an appropriate response when there's such a glaring error in the law," she said. "I think it's incumbent upon us to try to fix it."
Donnelly also acknowledged that some Democrats don't want to touch the healthcare law either because they oppose certain specific changes or because they simply don't want to invite a renewed debate over ObamaCare.
Donnelly voted for the Affordable Care Act, which passed while he was in the House.
Some liberal Democrats say setting the employer mandate at 30 hours per week was the best way to ensure that people who work 30 to 40 hours per week have access to healthcare.
Others, though, have said they would consider to a change. Sen. Tom HarkinTom HarkinGrassley challenger no stranger to defying odds Clinton ally stands between Sanders and chairmanship dream Do candidates care about our health or just how much it costs? MORE (D-Iowa) told The Hill last month that he would be "open to looking at it."
Moving the threshold to 40 hours would likely add more costs for the federal government, which would be paying for more subsidies to help people cover the cost of insurance they buy on their own.
But, Collins said, the status quo also costs the government money. As employers reduce workers' hours, their pay — and thus their taxes — decreases.
Businesses are "baffled by this definition," Collins said, adding that other federal laws generally define a full-time employee as someone who works at least 40 hours per week.
The senators also sent a letter to Obama on Wednesday asking the administration not to enforce the employer mandate's penalties in 2014, to give businesses more time to adjust to the new requirements.
"Give them the time to figure it out," Donnelly said.