Eighteen House Democrats voted with Republicans Thursday to change the definition of full-time work as defined by the Affordable Care Act, a sign that the recent success of the health law still hasn't erased concerns among some Democrats facing reelection this year.

The bill approved Thursday would change the law's definition of full-time work from 30 hours a week back to 40 hours a week, a move that Republicans say is necessary as employers continue capping the hours of part-time workers in anticipation of the law's employer insurance requirement.

Rep. Todd Young (R-Ind.), lead sponsor of the measure, said that those reductions in work hours are unfairly targeting lower-wage workers that the law was designed to help.

"These are cafeteria workers, these are substitute teachers, these are adjunct professors," he said this week. "These are folks who help us in terms of serving at restaurants. On balance, these are folks who can least afford to see a cut in their take-home pay. And so we want to restore the 40 hour work week."

Ahead of the vote, Republicans repeatedly cited the concerns of the service industry, a job sector that has continued hiring despite the fragile economic recovery. The American Hotel and Lodging Association complained Thursday that the law is affecting the already-complicated process of scheduling workers to staff hotels 24 hours a day. And the National Restaurant Association, a powerful Washington lobby, warned lawmakers Thursday that changing the law's work rules "would help avoid any unnecessary disruptions to employees’ wages and hours, and would provide significant relief to employers."

Thursday's vote marked the 55th time the GOP-led House has voted to scale back all or parts of the law -- and it won't be the last. When the White House announced Monday that more than 7 million Americans enrolled for health-care coverage, Speaker John A. Boehner (R-Ohio) vowed that "House Republicans will continue to work to repeal this law."

President Obama threatened to veto the House bill, citing a recent report by the nonpartisan Congressional Budget Office that said about 1 million people would lose employer-backed coverage and the number of uninsured would climb by almost 500,000 if the law's work definitions were changed. And most congressional Democrats, buoyed by the new enrollment figures, said that Republicans were merely bending to the will of companies who are using the new law as an excuse to cut workers' hours and pay.

"It is not Obamacare that decides how much somebody works, it’s the person who runs the company," Rep. James McDermott (D-Wash.) said Thursday.

The law passed 248 to 179. All voting Republicans were joined by 18 Democrats, most of whom have voted repeatedly in recent months to approve GOP-backed measures that would repeal all or parts of the health law. The group includes Democrats most at risk of losing reelection this year, including Reps. Ron Barber (D-Ariz.), Ami Bera (D-Calif.), Patrick Murphy (D-Fla.) and Nick Rahall (D-W. Va.).

But in a sign that at least some nervous Democrats may now be more willing to defend the law, the number of Democratic votes in support was less than the almost-four dozen who have voted with Republicans in recent months on other bills that would strip apart the law.

Even though the House measure has little hope for consideration in the Democratic-controlled Senate, some Democratic senators signaled this week that they are open to discussing ways to tweak the law's work rules. Citing the concerns of restaurateurs, Sen. Richard Durbin (D-Ill.), the chamber's second-ranking Democrat, told reporters Wednesday that “If [House Republicans] want to sit down in a constructive bipartisan discussion on that issue, I want to be at the table."

"There are legitimate concerns expressed in many industries, including restaurants, about how we’re going to define a full-time employee," Durbin said later. "Let’s sit down and work on it constructively."

Part of the way the Senate might take up the issue is as part of a forthcoming debate over raising the minimum wage. Sen. Susan Collins (R-Maine), a key GOP negotiator, said she is hoping to reach a bipartisan compromise to raise the federal minimum wage, but not at the levels being sought by Democrats. As part of her compromise, Collins said she would seek to raise the wage at a rate less than the $10.10 hourly rate that Democrats propose, include some small-business tax credits,  and an agreement to change the Affordable Care Act's full-time work definition back to 40 hours per week.

Democrats have yet to schedule a vote on the minimum wage increase, but Senate Majority Leader Harry M. Reid (D-Nev.) told reporters Thursday he is not currently seeking to negotiate with Republicans on a potential compromise.