The US Senate has started debating Republican proposals to repeal and replace President Barack Obama's signature healthcare policy.
Senators rejected the first of the plans under consideration - many votes on the issue are expected this week.
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. He says the system is "torturing" Americans.
On Tuesday night, the Senate began the debate-and-vote process, which is expected to last a number of days.
Nine Republicans voted against the first amendment - to repeal and replace Obamacare - and it failed to pass. The proposal, put forward by Senate Majority Leader Mitch McConnell, would have cut health insurance and subsidies for lower income groups.
Earlier, 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.
He secured a victory on Tuesday when the Senate agreed to allow the debate on health care legislation reform to go forward, but only after Republican Vice-President Mike Pence cast the tie-breaking vote in support of the bill.
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.
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.