US Supreme Court judges have signalled that they saw no procedural barrier to reaching the heart of the dispute over President Barack Obama's healthcare law that requires most Americans to buy insurance or pay a penalty.
During nearly 90 minutes of oral arguments, the judges voiced doubt that a US tax law requiring people to pay first and litigate later should delay the legal challenge.
At the core of the law, signed by Mr Obama in 2010, is a requirement that people obtain health insurance by 2014 or pay a penalty.
The question today was whether people can challenge this so-called individual mandate before paying the penalty and seeking a refund.
Several judges asked sceptical questions about whether the penalty was indeed a tax.
If not deemed a tax, then the justices could move forward to decide the merits of whether the law was constitutional.
"Here, they did not use that word tax," liberal judge Stephen Breyer said, referring to politicians who crafted the legislation in Congress.
Another liberal Democratic appointee to the court, Justice Ruth Bader Ginsburg, also expressed scepticism. "This is not a revenue-raising measure," she said.
Conservative Justice Antonin Scalia was also among those justices who suggested by his questions that allowing the case to go forward would not broadly undercut federal tax policy.
"There will be no parade of horribles," he said, noting that lower court judges would be able to determine when to make exceptions to the usual rules governing general tax penalties and law.
The nine judges, five appointed by Republican presidents and four by Democratic presidents, convened for the first 90 minutes of six hours of planned arguments over three days and they are expected to rule on the case by late June.
The law, intended to transform healthcare for millions of people in the US, has generated fierce political debate.
Republican presidential hopefuls and their colleagues in Congress have vowed to roll back the law, which they say will financially burden states, businesses and individuals.
Hundreds of supporters and opponents marched outside the Supreme Court building across from the US Capitol.
People lined up 72 hours in advance to try to get one of the few seats open to the public.
