The day had been designed as a big symbolic win for conservatives. Trump and House Republican leaders planned the vote on the seventh anniversary of former Democratic President Barack Obama signing his namesake healthcare law, formally known as the Affordable Care Act, which became a prime target for Republicans.
Some Republican conservatives felt the new Trump-backed healthcare plan, formally called the American Health Care Act, did not go far enough, while some moderates worried the legislation would hurt their constituents, so the bill was postponed indefinitely as negotiations continued into the evening.
The White House has insisted there is no "Plan B" if the bill fails.
"I am desperately trying to get to ‘yes’ and I think the president knows that. I told him that personally," said North Carolina Congressman Mark Meadows, the chairman of a group of conservatives known as the Freedom Caucus, which has been critical of the bill.
"We're going to get to the finish line," Meadows told reporters.
Trump, who has been working the phones all week and bringing lawmakers into the Oval Office to talk, met with the Freedom Caucus on Thursday morning, and was set to later meet with moderate Republicans, known as the 'Tuesday Group.'
As the healthcare drama unfolded on Capitol Hill, Trump took a break from the negotiations to talk about the issue and hang out with some truckers, climbing into the cab of one long-haul transport truck parked on the back driveway of the White House, and blowing the horn a few times.
He told reporters the vote would be close but remained optimistic. "I think we’re doing well. We’ll find out in about three hours," he said, just as reports began to surface that the vote had been postponed.
Even if the bill does eventually get approval from the House, the legislation faces a potentially tough fight in the Republican-controlled Senate.
The House and Senate had hoped to deliver a new healthcare bill to Trump by April 8, when Congress is scheduled to begin a two-week spring break.
MARKET TURNED RED
The vote was seen by financial markets as a crucial test of Trump's ability to work with Congress to deliver on his agenda.
Uncertainty over the bill rattled financial markets this week. U.S. stock markets rose steadily in recent months on optimism over a pro-business Trump agenda but fell back sharply on Tuesday as investors worried that failure to push through the healthcare bill could postpone other Trump priorities like tax cuts and infrastructure spending.
"The continued divisions within the Republicans will, at the margin, raise concerns about the prospects for the pro-growth legislation that the White House plans to subsequently submit to Congress, including tax reform," said Mohamed El-Erian, chief economic adviser at financial group Allianz.
In the last 30 minutes of trading on Thursday, the benchmark S&P 500 stock index dipped as the first headlines moved about the vote delay. It closed down very slightly.
"Delay on healthcare equates to delay on tax cuts. That is why the market turned red when the news flow suggested they didn't have a deal," said David Kotok, chairman and chief investment officer at Cumberland Advisors, a money management firm.
But others said it was too soon to panic.
"If this thing gets materially delayed or if we get a 'no' vote, we're going to see a horrific market reaction. But if they vote in the morning and it passes, we'll have a hell of a rally," said Jake Dollarhide, chief executive officer of Longbow Asset Management in Tulsa, Oklahoma.
The delay in the house vote is likely to extend the ups and downs in the stocks of some hospital groups and health insurers.
Trump and Republicans had campaigned on a promise to repeal and replace Obamacare, passed in 2010.
Republican control of the White House, the Senate and the House has given the party a chance to achieve what it has long aspired to do - overturn a law they cast as too invasive and too expensive.
Obamacare aimed to boost the number of Americans with health insurance through mandates on individuals and employers, and income-based subsidies. Some 20 million Americans gained insurance coverage through the law.
The House replacement plan would rescind the taxes created by Obamacare, repeal a penalty against people who do not buy coverage, slash funding for the Medicaid program for the poor and disabled, and modify tax subsidies that help individuals buy plans.
The Republican plan got more unwelcome news late on Thursday from the non-partisan Congressional Budget Office, which said that the latest version of the bill would reduce the U.S. budget deficit by less than the original version. The smaller deficit reduction was expected due to tweaks made this week to try to build support for the bill.
But the CBO said the revised plan would still have the same impact as the original bill on the level of uninsured Americans, with 24 million fewer people having medical coverage by 2026.
The Republicans have a majority in the House but because of united Democratic opposition, can afford to lose only 21 Republican votes. As of Thursday morning, NBC News said that 30 Republicans had planned to vote "no" or were leaning that way.
With this delay, House of Representatives Speaker Paul Ryan and his Republican leadership team will continue to search for ways to alter the legislation and bring it to a vote.
Graphic on Obamacare and Republican healthcare bill (tmsnrt.rs/2n0ZMKf)
Graphic on shifting positions in the U.S. Senate on Republican healthcare bill (tmsnrt.rs/2mUE4Xf)
Graphic on poll on Americans' views of the Republican healthcare bill (tmsnrt.rs/2n7f3e4)