Shortly afterwards, President Trump tweeted, "As I predicted all along, Obamacare has been struck down as an UNCONSTITUTIONAL disaster!"
US Senate Democratic leader Chuck Schumer said: "If this bad ruling is upheld in the higher courts, itwill be a disaster for tens of millions of American families".
In February, Attorney General Paxton and Wisconsin Attorney General Brad Schimel led a 20-state coalition lawsuit challenging the constitutionality of Obamacare, explaining that Congress rendered all of Obamacare unconstitutional by doing away with the tax penalty in Obamacare's individual mandate when it enacted President Trump's tax overhaul.
The ruling doesn't go into effect immediately, and is nearly certain to be appealed by state attorneys general who are defending the law.
The administration urged the court to declare those provisions invalid beginning January 1, 2019.
Trump urged Republican Senate Majority Leader Mitch McConnell and likely incoming Democratic Speaker of the House Nancy Pelosi and to "pass a STRONG law that provides GREAT healthcare". "Pending the appeal process, the law remains in place", the statement said, according to The New York Times. She said the House "will move swiftly to formally intervene in the appeals process to uphold the life-saving protections for people with pre-existing conditions and reject Republicans' effort to destroy the Affordable Care Act". They argued the judge should spare the rest of the law, which includes Medicaid expansion, the employer mandate, health exchanges, premium subsidies and federal health-care reimbursement rates for hospitals.
The White House said in a statement it expects the case to be appealed to the Supreme Court. If the entire health law is invalidated, popular provisions that benefit Medicare beneficiaries and people with employer coverage would also be scrapped.
Ex-Trump lawyer Michael Cohen jailed for 36 months
Legal experts said Cohen could get his sentence reduced if he strikes a deal with prosecutors to tell them more. Earlier this week, Trump compared his situation to one involving President Barack Obama's 2008 campaign.
Melania Slams 'Opportunists' Who 'Focus On Gossip' To 'Advance Themselves'
First lady Melania Trump made history during her visit with military service members on Wednesday by flying in an Osprey aircraft. Her skills aren't confined to print, and she appeared in a commercial for Aflac, an American insurance company, in 2005.
Chiefs seek 10th straight win vs Chargers, AFC West title
So, they used some new (and old) faces to complete some of their most important plays. Once Williams' catch was confirmed, the Chargers went for two and got it.
While O'Connor's ruling marks an uncertain moment for the millions of people who rely on Obamacare for health insurance coverage, it won't immediately take effect.
The five justices who upheld the health law in 2012 in the first major case - Chief Justice John Roberts and the court's four liberals - are all still serving.
The protections for people with preexisting conditions was one of the top health issues in the midterm elections in November.
US District Judge Reed O'Connor's 55-page ruling came in a lawsuit filed by several Republican state attorneys general opposed to the federal government health plan, known officially as the Affordable Care Act.
The ACA has been a battle between Democrats, who favour the law, versus Republicans who have voted repeatedly to repeal it.
California and Democratic officials in 14 states, along with the District of Columbia, won permission to defend ACA in the Fort Worth case when the Trump administration sided with the states seeking to dismantle it.
O'Connor is a conservative Republican appointee who has previously blocked other Obama-era policies.