In a big package of tax and spending legislation that Congress is likely to approve this week, Republicans have forced President Obama to swallow three changes that undermine his signature health care law, including a two-year delay of a tax on high-cost insurance plans provided by employers to workers.

In an interview on Wednesday, Mr. Obama’s first budget director, Peter R. Orszag, a leading supporter of the Cadillac tax, said, “The two-year delay is likely to be equivalent to repeal of the tax because people will expect it to be deferred again and again.”

The House reached a deal late Tuesday on a $1.1 trillion spending bill and a huge package of tax breaks. Throughout Tuesday, major components of the spending legislation appeared to be falling into place, including an agreement to alter major provisions of the Affordable Care Act, delaying a planned tax on high-cost health insurance plans and suspending a tax on medical devices for two years. Lawmakers also agreed to delay the Cadillac tax on high-cost employer-sponsored health plans by two years, originally scheduled to take effect in 2018.

Sen. Marco Rubio’s efforts against the so-called risk corridor provision of the Affordable Care Act, which was intended to help insurance companies cope with the risks they assumed when participating in the health care law’s new marketplaces, has shown the effectiveness of quiet legislative sabotage. Because of Rubio’s efforts, the administration says it will pay only 13% of what insurance companies were expecting to receive this year.

Over the last few years, Kentucky captured the nation’s attention by wholly embracing the health care law and expanding Medicaid under the Affordable Care Act. Now, with Governor-elect Matt Bevin promising to “repeal the expansion as it currently exists,” Kentucky may become a laboratory for the kind of rollback that the law’s opponents have so far only dreamed of.

The latest turmoil in health insurance marketplaces created by the Affordable Care Act has emboldened both conservatives who want to shrink the federal role and liberals who want to expand it. UnitedHealth Group announced last week that it may pull out of the marketplaces in 2017 after losing money this year. This followed the collapse of 12 of the 23 nonprofit insurance cooperatives created with federal loans under the health law. In addition, insurance markets in many states are unstable. Premiums are volatile and insurers say their new customers have been sicker than expected.

There are 499 markets for Obamacare plans in the United States. In 89 of them, the insurance company that offered this year’s best deal in the “silver” category will not be returning for 2016. The New York Times has created an interactive map showing in what areas of the United States this is the case.

For many consumers, switching health insurance plans has become an unwelcome ritual, akin to filing taxes, that is time-consuming and can entail searching for new doctors and hospitals each year. Gail Galen, 63, is preparing to leap to her third insurer in three years as she braces for another round of shopping on the federal insurance marketplace. “Every year I feel like I’m starting all over again, and I just dread it,” said Galen. “My stress level just shoots up.”

Starting in January, the Affordable Care Act will require businesses with 50 or more full-time-equivalent employees to offer workers health insurance or face penalties that can exceed $2,000 per employee. The health care law’s employer mandate, a provision that business groups fought against fiercely, is intended to make affordable health insurance available to more people by requiring employers to bear some of the cost of providing it. For some business owners on the edge of the cutoff, the mandate is forcing them to weigh very carefully the price of growing bigger.

The Obama administration officials are touting low premiums available during open enrollment on Healthcare.gov, but for many new patients receiving coverage under the ObamaCare exchanges, the sticker shock of sky-high deductibles leaves them just as vulnerable as before they were covered. The New York Times found that in many states, more than half the insurance plans offered on the federal exchanges had deductibles of $3,000 or more.

The financial failure of more than half the nonprofit health insurance companies created under the Affordable Care Act has handed Republicans a new weapon in their campaign against the health law, thrown the Obama administration on the defensive once again and left more than a half-million consumers in the cold.