On Tuesday, Sen. Elizabeth Warren (D-Mass.) blamed Republican lawmakers for undermining the U.S. health care system through their efforts to repeal the Affordable Care Act (ACA).

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In a series of tweets, Warren said President Donald Trump and the GOP "have worked around the clock to sabotage health care," effectively forcing the first major insurer, Humana, to drop out of the ACA market in 2018 on Tuesday.

"When you shut down ads, fewer people sign up," the senator wrote. "Now Humana is pulling out of the exchanges. They don’t have enough healthy young [people] enrolled."

In the lead-up to the Jan. 31 enrollment deadline for the ACA, the administration pulled about $5 million worth of television and radio ads. Some have speculated the the ad removal — in addition to repeated promises to repeal the health care law — contributed to lower-than-projected enrollment number for 2017.

The effectiveness of the ACA depends, in large part, on a diverse pool of enrollees — and having a healthy number of young people signing up is critical in mitigating costs for insurers. But repeated promises by Republican lawmakers to dismantle the health care law, and Trump's signing of an executive order designed to restrict enforcement of the law's provisions, created uncertainty surrounding the ACA.

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About 9.2 million Americans signed up for ACA coverage by the deadline in 2017, compared to 9.6 million in 2016. But what's striking is that in the last two weeks before the 2016 deadline, about 670,000 people signed up — compared to about 380,000 sign-ups in the last two weeks before the 2017 deadline, according to the Department of Health and Human Services (DHS).

Republicans have consistently campaigned on repealing the ACA, also known as "Obamacare." Recently, some have have complained about rising premium costs under the ACA, pointing to the fact that several insurance companies pulled out of the ACA market following the law's implementation.

"Based on our initial analysis of data associated with the company's health-care exchange membership following the 2017 open enrollment period, we continue to see further signs of an unbalanced risk pool," Humana CEO Bruce Broussard said during a conference call Tuesday. "Therefore, the company has decided that it cannot continue to offer this coverage for 2018."

Trump interpreted the news as evidence of the ACA's failure in a tweet, pledging again to develop a replacement plan that will "save healthcare for ALL Americans."

Warren attributed the "unbalanced risk pool" to the Republican's repeal efforts, emphasizing the reluctance of young people to enroll in the health care exchange. She said Republicans "should stop these reckless actions to hurt the ACA before they hurt even more people who need health coverage."

It's unclear what drove the lower enrollment numbers in 2017, but Axios' David Nather noted that repeal talks and the pulling of ads "may have had an impact, since the pace of enrollment had been ahead of last year's until mid-January."