WASHINGTON — The Obama administration announced on Monday that it would postpone enforcement of a federal requirement for medium-size employers to provide health insurance to employees and allow larger employers more flexibility in how they provide coverage.

The delay is the latest in a series of policy changes, extensions and clarifications by the administration, and it drew a new round of criticism from congressional Republicans, whose scorching attacks on the law have become a central theme in many of this year’s midterm election campaigns.

The “employer mandate,” which was originally supposed to take effect last month, had already been delayed to Jan. 1, 2015, and now the administration says that employers with 50 to 99 employees will not have to comply until 2016 — allowing Democrats to placate business concerns and pushing the issue well beyond this year’s midterm elections.

“Today’s final regulations phase in the standards to ensure that larger employers either offer quality affordable coverage or make an employer responsibility payment starting in 2015,” said Mark J. Mazur, the assistant Treasury secretary for tax policy. The purpose of the penalty, he said, is to help offset the cost to taxpayers of providing coverage or subsidies to people who cannot get affordable health insurance at work.

Under the law, employers with fewer than 50 full-time employees are generally exempt from the requirement to offer coverage.

The administration described the new policy as a form of “transition relief” to help employers adjust to requirements of the 2010 health care law.

But congressional Republicans jumped on the delay as only the latest maneuver by the Obama administration to sidestep the health care law’s legal requirements for political gain. Republicans denounced the unilateral move as a violation of the law and called on the White House to throw out all of the Affordable Care Act’s coverage mandates.

“The White House seems to have a new exemption from its failed law for a different group each month,” said Senator Mitch McConnell of Kentucky, the Republican leader, who is in a competitive race himself. “It’s time to extend that exemption to families and individuals — not just businesses.”

But the House Democratic leader, Representative Nancy Pelosi of California, praised the White House, saying the final rules showed “the administration’s commitment to smoothly implement the Affordable Care Act.”

Coming on the heels of a government analysis of the law’s impact on the work force, the delay is likely to breathe new life into the Republican effort to make the health care law the central issue in the coming midterm elections. Several lines of attack, which started with the disastrous rollout of the Healthcare.gov website in October and shifted to a wave of insurance cancellation notices in November, have largely run their course as the website’s problems came under control and canceled policies were replaced.

“Once again, the president is rewriting law on a whim,” said Speaker John A. Boehner. “If the administration doesn’t believe employers can manage the burden of the law, how can struggling families be expected to?”

That theme was echoed by many Republicans, who say it is unfair for the White House to grant a dispensation to employers but not to individuals and families. Democrats see the individual mandate as more important to the operation of the law.

 But lawmakers from both parties have raised questions about unilateral actions by the president to waive or delay various provisions of the law. J. Mark Iwry, deputy assistant Treasury secretary for health policy, said the administration had broad “authority to grant transition relief” under a section of the Internal Revenue Code that directs the Treasury secretary to “prescribe all needful rules and regulations for the enforcement” of tax obligations. This authority has often been used to postpone the application of new laws that would cause “unreasonable administrative burdens or costs” to taxpayers, Mr. Iwry said.

Under the law, larger employers may be subject to tax penalties if they do not offer “minimum essential coverage” to employees who work at least 30 hours a week, on average.

Larger companies have, for many years, been more likely to offer coverage than smaller ones.

The Treasury said that companies with 50 to 99 employees accounted for 7 percent of the private sector work force, while businesses with 100 or more workers accounted for 66 percent. Most companies with 100 or more employees already offer health benefits to at least some of their workers. Small businesses with fewer than 50 employees account for nearly 28 percent of private sector employees, but 96 percent of all private employers, the Treasury said.

Paul M. Hamburger, a lawyer who advises employers at the Proskauer law firm, said: “The bottom line is that Obamacare is not going away. The administration is providing another one-year delay, until 2016, for some employers. But they have to take the law seriously and figure out ways to comply. The administration did not provide the relief sought by employers in some high-turnover industries like restaurants and retail.”

In response to concerns expressed by lawmakers from both parties, the administration said that local government agencies would generally not have to provide health insurance to “bona fide volunteers” who work as firefighters or emergency medical technicians.

The administration said it was still trying to figure out how to count the hours worked by certain types of employees, including part-time college instructors who are paid for teaching a certain number of classes or courses. Until guidance is issued, Treasury officials said, colleges may assume that such “adjunct faculty” members spend 75 minutes a week outside the classroom, preparing lectures or grading examinations, for each hour teaching in the classroom.

Federal officials said they were also providing some relief to employers of seasonal employees. Those who normally work a half-year or less will generally not be considered full-time employees, the administration said.