Rudy Figueroa (R), an insurance agent from Sunshine Life and Health Advisors, speaks with Marvin Mojica as he shops for insurance under the Affordable Care Act at a store setup in the Mall of Americas on November 1, 2017 in Miami, Florida.

Obamacare sign-ups on the federal health insurance marketplace fell by 20.4 percent in the first two weeks of this enrollment season compared to last year, according to new federal data.

The tally is being closely watched because fiscal 2019 will be the first year since 2013 in which Americans will not be penalized for failing to have some form of health insurance coverage.

A total of 1,176,232 people in the first 10 days of open enrollment chose a plan for the 2019 coverage year on HealthCare.gov, the federal health insurance exchange that serves much of the United States, according to data released Wednesday by the Centers for Medicare and Medicaid Services.

That compares with 1,478,250 consumers who selected their coverage through the exchange during the first two weeks, or 11 days, last year, according to federal data released at the time.

More than 901,300 existing customers renewed their coverage on the marketplace, while 274,913 new consumers chose an insurance plan on HealthCare.gov. Roughly 8.8 million people enrolled in Obamacare during last fiscal year.

Open enrollment began Nov. 1 and will run until Dec. 15 for most states. People who do not sign up for an Obamacare plan by the end of open enrollment will not be able to obtain coverage until the fall of 2019, unless they have a so-called qualifying life event.

This is the first enrollment season since Congress repealed Obamacare's individual mandate.

That mandate imposes a tax penalty on consumers who are uninsured. The mandate, which remains in effect for 2018, was a key part of the Affordable Care Act legislation, as Obamacare is formally known. The mandate is the greater of $695 person per adult or 2.5 percent of household income.

Without the mandate, Obamacare exchanges could see fewer sign-ups from younger, healthier people who feel they may not need coverage, said Judy Solomon, a senior fellow at the Center on Budget and Policy Priorities, a Washington think tank.

But, Solomon added, "We're not hearing anything to makes us think that demand is off."

However, in addition to the mandate's repeal, another factor that could depress enrollment for 2019 plans is the move by the Trump administration to allow people to remain longer in less-expensive short-term health plans.

The Obama administration had restricted the use of short-term plans — which as a rule offer less comprehensive coverage of benefits — to three months.

But the Trump administration is allowing people to stay in short-term plans for up to 12 months. And it is allowing consumers to renew their coverage in such plans twice.

Some analysts predict those less-expensive plans could attract healthier customers than people who opt for the more expensive, and more comprehensive Obamacare plans.

That could in turn lead insurers who sell Obamacare coverage to increase the prices of their plans to offset the costs from providing benefits for their customers.

However, the Obamacare exchanges are seeing some stability after years of regulatory uncertainty, which had previously led to price hikes on plans.

Insurers such as Cigna are starting to turn a profit for their Obamacare products.

And this enrollment season has seen the return of a number of insurers to the Obamacare marketplaces.

"I think people are really understanding the value of health insurance and getting used to how the subsidies work," said Solomon of the CBPP.

—CNBC's Bertha Coombs and Dan Mangan contributed to this report.