Two Republican senators are demanding that the Obama administration provide an update on the number of Obamacare enrollees, after several indications that the number could be far lower than the number claimed by officials back in May.

Sens. Lamar Alexander (R-Tenn.) and John Barrasso (R-Wyo.) asked Centers for Medicare & Medicaid Services Administrator Marilyn Tavenner for updated figures by September 9. Their letter to Tavenner noted that the last update the administration gave was on May 1, when it claimed 8.1 million enrollees.

Obamacare CMS health care Marilyn Tavenner enrollment data

The last enrollment update on Obamacare was back in May, and there are signs that thousands of people may have dropped out of Obamacare since then. (AP Photo/Jon Elswick, File)

“Since the last enrollment report issued by CMS, numerous investigations raise questions about the accuracy of the enrollment figures trumpeted by the administration in May,” they wrote.

Even when the 8.1 million number was announced, many Republicans raised questions about whether enrollment only counted people who bought a plan, or a wider group of people who signed up but had yet to pay.

In the nearly four months since then, a major looming question is whether all 8.1 million people “enrolled” in May are actually paying insurance premiums. Earlier this month, reports began emerging that many insurance companies were seeing people drop out of their plan after they first signed up.

For example, Investor’s Business Daily reported that Aetna, the country’s third-largest insurer, had just 600,000 people paying into an Obamacare plan, after originally signing up 720,000 people. The publication said that Aetna believed the number could drop further by the end of 2014.

Another insurer, Cigna, said it expected to lose about 20,000 enrollees.

In addition to raising questions about current enrollee totals under Obamacare, these reports have led to related questions about the health status of those still enrolled. One key to making Obamacare work, many experts say, is ensuring enough healthy people are signed up to offset costs associated with treating those who need care.

Alexander and Barrasso’s letter cited several other factors that could be leading to lower enrollment numbers. One is a recent report from the Department of Health and Human Service’s Office of Inspector General, which found that Obamacare was having problems determining whether people are eligible for health care subsidies.

If people qualify for fewer subsidies, that could make it harder for them to afford health plans, which could lead to another drop in enrollment.

The senators noted a related issue, which is a July report from the Government Accountability Office saying that GAO created 12 fictitious applicants, and that 11 of them were able to sign up for coverage. That could be a sign that the 8.1 million enrollment number is inflated, if it’s found later that many applicants are not legitimate.

“Finally, your own agency recently acknowledged that over 300,000 people have not responded to problems found with their immigration and citizenship status,” the letter added. “Despite repeated efforts to contact these individuals, they have not provided the documentation to prove that they can legally receive plans through the exchanges.”

The next open enrollment period starts in mid-November, and the two senators said it is “imperative” that CMS provide updated enrollment numbers before then.

Specifically, they asked for updated enrollment numbers as of August 15 and how many people have signed up but then later failed to pay the first month’s insurance premium. The letter also asked how many paid their first month’s premium, but then failed to pay premiums in later months.