WASHINGTON — For the second time in a year, the Obama administration is giving certain employers extra time before they must offer health insurance to almost all full-time workers.

Under new rules announced Monday by Treasury Department officials, employers with 50 to 99 workers will be given until 2016 — two years longer than originally envisioned under the Affordable Care Act — before they risk a federal penalty for not complying.

Companies with 100 workers, instead of being required in 2015 to offer coverage to 95 percent of full-time workers, can avoid a fine by offering insurance to 70 percent of them next year.

How the administration would define employer requirements has been one of the biggest remaining questions about the way the 2010 health-care law will work in practice. By providing dual phase-ins for employers of different sizes, administration officials have sought to lighten the burden on the small share of affected employers that have not offered insurance in the past.

Many across the ideological spectrum view the delays as an effort by the White House to defuse another health-care controversy before the fall midterm elections. The new postponements won over part, but not all, of the business community.

Congressional Republicans seized on the announcement as the latest justification for scrapping the health-care law. They said it is unfair to delay rules for businesses, but not individuals.

“If unilateral delays were an Olympic sport, the White House would sweep the gold, silver, and bronze,” House Energy and Commerce Committee Chairman Fred Upton, R-Mich., said in a statement.

Originally, the employer mandate — which affects companies employing 72 percent of all Americans — was to have gone into effect Jan. 1.

A senior administration official said the Treasury Department decided to allow medium-size businesses more latitude because they “need a little more time to adjust to providing coverage.”

The law says that anyone who works 30 hours or more is a full-time employee, and it compels many employers to offer affordable insurance to those workers and dependents. It defines affordable as premiums of no more than 9.5 percent of an employee’s income, and employers must pay for the equivalent of 60 percent of the actuarial value of a worker’s coverage. Businesses that fail to do so will face a fine of up to $2,000 for each employee not offered coverage.

Under the health-care law, employers with fewer than 50 workers do not have to offer insurance. They will be allowed to buy health plans through new marketplaces available next fall.

Until now, the government had not spelled out important details of the health care law. Nor had it defined exactly what insurance benefits must be covered by employer-sponsored health plans.

Administration officials said Monday that they will issue a separate set of rules in coming weeks covering questions about how employers must report workers’ insurance status.

Senior administration officials said the latest postponement will have little impact, because the vast majority of employers have fewer than 50 workers.

But employees of bigger companies represent a much larger fraction of the U.S. workforce, with more than seven in 10 at companies and organizations that employ 100 or more people.

Trade associations — which represent many of the U.S. businesses affected by the new requirements — had a mixed reaction to the rules.

The changes pleased fire companies, which feared they’d be required to offer insurance to volunteer firefighters. The rule also said that educators who have summers off are nonetheless to be treated as full-time workers entitled to be offered coverage. And seasonal workers — such as farmworkers or extra department-store hires around Christmastime — are considered full time only if they work for at least half the year.