John McCain entered the Senate to cast the deciding vote
The US Senate has voted to start debating a new Republican healthcare bill in a major step forward for President Donald Trump's attempts to repeal his predecessor's signature law.
In a tight vote, several Republicans previously opposed to aspects of the plan changed their minds.
Prior attempts to replace Obamacare have collapsed in recent weeks due to divisions in the Republican party.
President Trump had made scrapping the policy a key campaign pledge.
Before the vote took place, Mr Trump had warned his party's senators of the repercussions of not pushing through the measures to repeal and replace Obamacare, known formally as the Affordable Care Act.
The Republican party, which needed a majority for the motion to go ahead, secured 51 votes after Vice-President Mike Pence cast a tie-breaker in support of the legislation.
Senator John McCain, who was recently diagnosed with an aggressive brain tumour, received a standing ovation as he returned to Congress to cast his "Yes" vote.
President Trump tweeted his thanks to the Arizona senator for playing "such a vital role" in the vote.
"Congrats to all Rep.We can now deliver grt [great] healthcare to all Americans," he wrote.
What happens next?
It remains unclear what measure senators will now debate and vote on.
There appear to be two choices - either a repeal-and-replace bill that has already struggled to win support across the party, or a bill that enacts repeal with a two-year delay, in the hope of finding agreement before that time elapses.
But senators have also considered a "skinny bill", a far narrower measure that would scale back some of the more controversial elements in an effort to get a wider consensus.
Still a long way to go
Anthony Zurcher, BBC News, Washington
If Senate Majority Leader Mitch McConnell wants to get healthcare reform through the Republican-controlled Senate, it's going to take every trick in the book.He used a few of them on Tuesday.
Some Senate Republicans are threatening to block a vote on procedural grounds?Then force the procedural vote without any clear understanding of what the final bill will look like.
That manoeuvre only delays the day of reckoning, however.Now differences within the Republican Party will have to be resolved on the Senate floor- a metaphorical tightrope act without a net.
Are there 50 votes for anything substantive - the original Senate bill, the modified Senate bill, the House bill or straight-up repeal without a replacement?That's a tall order.Mr McConnell could eventually break the reform bill into pieces, hoping that at least the popular provisions - like ending the requirement that all Americans obtain insurance - can win a majority.
If something - anything - passes, the next step is negotiations with the House to resolve differences in the bill.If a compromise is reached, it's back to the Senate and House to vote on it.
In other words, Republicans aren't out of the woods yet.Not by a mile.
What have Republicans proposed?
Republicans have long railed against Obamacare as government overreach, criticising the system for introducing government-run marketplaces, where premiums have risen sharply for some people.
The party's proposed alternative includes steep cuts to Medicaid, a healthcare programme for the poor and disabled.
And it removes Obamacare's individual mandate requiring all Americans to have health insurance or pay a tax penalty.
About 20 million people gained health insurance under former President Obama's Affordable Care Act.
The non-partisan Congressional Budgetary Office (CBO) found the bill would strip 22 million Americans of health insurance over the next decade.
If Republican senators elect to repeal key provisions of the law without immediately replacing it, the CBO estimates about 32 million consumers would lose insurance over the next 10 years.
Six months of Trump
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