The waiting room at Mary's Center in Washington, DC on February 24, 2014. Mary's Center is a non-profit health center for the underserved, uninsured or underinsured.

The number of adults without health insurance has increased 1.4 percentage points since the end of 2016, partially reversing the gains in health coverage seen since the implementation of Obamacare.

The nation's uninsured rate — which fell to a record low of 10.9 percent in the last half of 2016, is now 12.3 percent, according to a Gallup-Sharecare Well-Being Index survey released Friday.

That increase represents almost 3.5 million Americans who became uninsured in the time since Obamacare opponent Donald Trump was elected president and took office. And the uninsured rate is now the highest recorded since the end of 2014, when it stood at 12.9 percent.

Gallup's report on the findings said the uninsured rate for the third quarter of this year rose by 0.6 percentage point from the previous quarter.

That report warned that unless Trump and Congress take action to stabilize Obamacare markets, "the number of uninsured Americans likely will continue to rise."

The growth in the uninsured since late 2016 has been concentrated in middle-aged adults, racial minorities and low-income people, according to Gallup, whose survey is based more than 45,000 interviews with adults.

"Still, the uninsured rate remains well below its peak of 18 percent measured in the third quarter of 2013," according to Gallup's report.

That time frame was right before Obamacare marketplaces began selling subsidized individual health plans, and before implementation of the rule requiring most Americans to have some form of health coverage or pay a fine.

Despite Gallup's warnings about likely future increases in the uninsured rate, it is not clear that Trump or Congress will act to avert that from happening.

Trump last week cut off key federal payments to insurers, which will trigger higher Obamacare premiums. The president also signed an executive order that could further undercut Obamacare markets.

However, a bipartisan bill introduced in the Senate this week is seeking to shore up the Obamacare markets, at least in the short term.