However, Democrats including incoming Speaker of the House Nancy Pelosi vowed to fight to save former President Barack Obama's signature legislation.
While Collins pointed to data suggesting lower- and middle-income taxpayers were more likely to pay penalties for not having health insurance, advocates for the Affordable Care Act have pointed to a 2017 Congressional Budget Office analysis that forecast that the mandate's repeal would result in 13 million more uninsured Americans over the next decade and 10 percent increases in premiums most years over that timespan.
Obama clarified that the decision did not immediately strike down the law and that there was a long legal battle ahead.
Texas and an alliance of 19 states argued to the judge that they've been harmed by an increase in the number of people on state-supported insurance rolls.
If the appeals court - and Supreme Court - uphold Friday night's ruling, it could translate into an abrupt and disruptive turn in the health care of millions of Americans.
President Trump said it was a great ruling for our country. "If a majority of the House and a majority of the Senate say that this case should be overturned, it'll have a tremendous effect on the appeal", Schumer said.
"The GOP spent all previous year pretending to support people with pre-existing conditions while quietly trying to remove that support in the courts", Senate Democratic leader Chuck Schumer of NY said in a tweet Saturday.
"There is widespread support for protecting people with pre-existing conditions, and there's widespread opposition to the individual mandate", Collins said. President Donald Trump has pledged to repeal and replace Obamacare since the early days of his administration.
"This outrageous ruling threatens health coverage for millions of Americans ..." However, we're not there with O'Connor's ruling yet.More news: Celtic lose but scrape through to Europa League knockout stage
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Also at risk are provisions that affect the wider health insurance market, such as keeping adults on their parents' policies until age 26.
About 20 million people have gained health insurance coverage since the law passed in 2010 without a single Republican vote.
"There's really no American that's not affected by this law", said Yale law professor Abbe Gluck, who filed an amicus brief with other lawyers in the Texas case.
Midnight Saturday is the deadline to sign up for 2019 plans via HealthCare.org.
House Majority Whip Steve Scalise, a Louisiana Republican, said in a statement that "Democrats may have won the House by running a fear-mongering campaign, unfortunately their reckless legislating has not proven to be as effective for the American people who have to live under this failed system".
"We know what Congress' intent was in 2017 - that was to pull the individual mandate while keeping the rest of ACA intact", University of MI law professor Nicholas Bagley said. After Trump ordered the Justice Department to stop defending the health law, a coalition of ACA-supporting states took up the defense.
Since then, public opinion on the overhaul has shifted from mostly negative to generally favourable.
The protections for people with preexisting conditions was one of the top health issues in the midterm elections in November.
Despite Trump's celebration, his administration - via Seema Verma, administrator for the Centers for Medicare and Medicaid Services - was quick to emphasize that the current law remains in effect, for now.