WASHINGTON — Even as the Affordable Care Act remains a political flash point, new research shows it is dramatically improving poor patients' access to medical care in states that have used the law to expand their Medicaid safety net.

After just two years of expanded coverage, patients in expansion states are going to the doctor more frequently and having less trouble paying for it, according to the study. At the same time, the experience in those states suggests better access will ultimately improve patients' health as people get more regular checkups and seek care for chronic illnesses such diabetes and heart disease.

“The effects of expanding coverage will be an unfolding story over time,” said Dr. Benjamin Sommers, lead author of the study, published in the journal JAMA Internal Medicine. “But we are starting to see the kind of broad-based improvements that we would expect with better access.”

The health law, often called Obamacare, nevertheless continues to be a major point of dispute in national politics. And Republican politicians in many red states oppose Medicaid expansion, arguing that the program is unaffordable and ineffective.

GOP presidential nominee Donald Trump has pledged to repeal the law.

Democratic nominee Hillary Clinton has pledged to build on the existing law. At her nominating convention last month in Philadelphia, Democrats embraced the law more overtly than they have done in previous campaigns.

That reflects a view by some Democratic strategists that opposition to the law, while strong among core Republicans, has waned among other voters, and that the added coverage can mobilize support, particularly among low-income and minority voters.

Medicaid is a key pillar of the law's program for guaranteeing health coverage and has helped drive a historic drop in the nation's uninsured rate. Nationwide surveys suggest at least 20 million previously uninsured Americans have gained coverage since 2014.

The law makes hundreds of billions of dollars of federal aid available to states to extend Medicaid coverage to poor adults, a population that had been largely excluded from the government safety net program. Medicaid eligibility historically was limited to certain vulnerable populations, including low-income children, pregnant women, people with disabilities and the elderly.

The number of states that have expanded Medicaid under the law has slowly increased to 31, the most recent being Louisiana.

But continued GOP opposition has left nearly 3 million low-income Americans without health insurance in the 19 states that haven't expanded Medicaid, according to an analysis by the nonprofit Kaiser Family Foundation.

That is already having a profound effect, according to the new JAMA study, which builds on growing evidence showing a widening divide between states that are expanding access and those that aren't.

Sommers and other researchers at Harvard University have been tracking the effect of Medicaid expansion by surveying some 9,000 poor residents in Arkansas and Kentucky, both of which expanded Medicaid eligibility, and in Texas, which has rejected the expansion.

In Arkansas and Kentucky, the share of poor adults without health insurance plummeted between 2013 and 2015, from more than 40 percent in both states to 14 percent in Arkansas and less than 9 percent in Kentucky.

In Texas, by contrast, the uninsured rate dropped from 39 percent to 32 percent. Although Texas has not expanded Medicaid, state residents have been able to buy health insurance on the new insurance marketplaces that were also created by the law.

The new coverage in Arkansas and Kentucky dramatically improved poor patients' access to care and relieved financial strains, the survey shows.

The share of patients in the two states who had trouble paying medical bills fell more than 11 percentage points between 2013 and 2015, while in Texas, more patients reported medical bill problems in 2015 than in 2013.

Survey data from 2014 showed those improvements after just one year of the Medicaid expansion in Arkansas and Kentucky.

But with another year of data, researchers noted more evidence that the improved access is also allowing more poor patients to seek recommended medical care.

noam.levey@latimes.com