Demonstrators interrupted the start of the make-or-break vote with shouts of “kill that bill, don’t kill us” and “shame” as they stood in the visitors’ gallery. They were led out of the chamber but could still be heard.
Senators voted to formally open debate on legislation to repeal much of the Affordable Care Act, a key achievement of President Trump’s predecessor, Barack Obama.
The final tally was 51-50, with Vice President Mike Pence breaking the tie.
Mr Trump, who made getting rid of the legislation a key plank of his successful White House bid, had called on Republicans to “step up to the plate” on the issue after “seven years of talking”.
Senate Majority Leader Mitch McConnell urged colleagues in his party, which controls both chambers of Congress, to vote in favour of moving forward on the legislation.
Democratic leader Chuck Schumer urged senators to reject it, saying: “Turn back now before it’s too late.”