(Reuters) — The top White House adviser on health care said on Monday she did not anticipate how lengthy and persistent the political opposition would be to health care reform — U.S. President Barack Obama's signature domestic legislative achievement.

The administration had not anticipated opposition to the Patient Protection and Affordable Care Act of 2010 to drag on years after the law's passage, said Kathleen Sebelius, secretary of the U.S. Department of Health and Human Services.

Despite its goal to provide health coverage to more than 30 million uninsured Americans and improve the quality of medical care delivered across the country, many voters have balked over concerns the law will raise health care costs and increase government involvement in their personal decisions.

"The politics has been relentless and that continues," Ms. Sebelius said. "There was some hope that once the Supreme Court ruled in July and then once an election occurred there would be a sense that, 'This is the law of the land, let's get on board, let's make this work.' And yet we will find ourselves having state by state political battles."

One factor that contributed to the law's unpopularity for a large segment of the American public is the fact that its benefits have taken several years to impact people directly. Only in 2014 will many of its broadest benefits take effect, extending health coverage to millions more people.

At the same time, Republican governors and legislatures in states including Texas, Louisiana and South Carolina have refused to set up state-run exchanges where individuals would be able to buy insurance and have rejected the law's provision to allow more low-income people to enroll in Medicaid insurance.

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"It is very difficult when people live in a state where there is a daily declaration, 'We will not participate in the law,' for them to figure out whether they are going to benefit," Ms. Sebelius said at an event at the Harvard School of Public Health in Boston, presented in collaboration with Reuters.

Opponents of Medicaid expansion have argued that states could find themselves saddled with untenable financial burdens if the federal government later cut back its funding to the program.

Loudest among them has been the voice of Texas' Republican Gov. Rick Perry, who has described the Medicaid program as "a broken system."

Other prominent Republicans have sought to compromise; in Florida, Gov. Rick Scott has sought, so far unsuccessfully to persuade his legislative colleagues to accept the Medicaid expansion.

The law also faces continued opposition from corners of the $2.8 trillion U.S. health care industry.

Ms. Sebelius, addressing the issue of feared increases in health insurance premiums because of the law, said competition to provide a set of mandated basic benefits would help.

"For the first time ever in the history of the United States, they'll have to compete for service and customers, not by cherry-picking the market (and) trying to figure out who can only insure people who promise never to get sick," she said.

Ms. Sebelius is one of a handful of members of President Obama's cabinet to stay on for the president's second term in office.