Supporters of the Affordable Care Act participate in a "Save Obamacare" rally in Los Angeles, California on March 23, 2017.

Possible reforms or updates to the 2010 law have been a central focus of the Democratic presidential primary.

The 20 states and D.C. urged the top court in a petition to hear their appeal in its current term. The House filed a separate petition with the same request. If the court agrees to do so, a decision will come by the end of June, in the thick of the presidential campaign season.

A federal appeals court ruled last month that the individual mandate provision of the law is unconstitutional. But the court directed a lower court to decide whether the rest of the landmark health-care law should be tossed out.

A California-led coalition of other Democratic states, as well as the District of Columbia and the House of Representatives, asked the Supreme Court on Friday to review a lower court decision that cast doubt on the constitutionality of the Affordable Care Act, also known as Obamacare, in a pair of petitions.

"While the Trump Administration fights to strip access to healthcare, our coalition moves forward to defend it — because a pre-existing medical condition should never again disqualify you from receiving affordable healthcare," California's attorney general, Xavier Becerra, said in a statement.

President Donald Trump, via the Department of Justice, backed the coalition of Republican states that challenged the law. Texas led the suit, which was opposed by Democratic-led states and the House of Representatives, which gained a Democratic majority in the 2018 midterms.

The Supreme Court upheld Obamacare's core provisions in a landmark decision in 2012. Chief Justice John Roberts, who authored the court's opinion, reasoned at the time that the individual mandate was permissible under Congress' power to levy taxes.

The current case gained attention after Judge Reed O'Connor of the Northern District of Texas ruled in 2018 that the entire health law was unconstitutional because the individual mandate penalty had been essentially eliminated.

Congress set the penalty at $0 in 2017 as part of the Tax Cuts and Jobs Act, a sweeping tax bill that remains Trump's signature legislative achievement.

After the Democratic states lost their appeal before the 5th U.S. Circuit Court of Appeals, Becerra pledged to "move swiftly to challenge this decision because this could mean the difference between life and death for so many Americans and their families."

Health policy experts believed the appeals court decision would likely delay the Supreme Court hearing the landmark case until after the 2020 presidential election, with a ruling possible in 2021. The uncertainty surrounding the law contributed to capped stock gains for some health-care companies since the coalition of Republican states challenged the law.

The states argued that hearing the case on an expedited basis is necessary because of the uncertainty caused by the 5th Circuit's decision for patients, doctors, insurers and governments.

"The shadow cast by the decisions below may also negatively affect the health insurance market in future years by, for example, causing insurers to increase premiums or withdraw from the Exchanges altogether," they wrote in a motion filed alongside their petition.

About 8.3 million people signed up for 2020 health plans on the HealthCare.gov website this enrollment season. More than 12 million Americans receive coverage through Obamacare's Medicaid expansion.