A vote in the US House of Representatives on a bill to repeal and replace "Obamacare" has been delayed, in a stinging setback for President Donald Trump and House Speaker Paul Ryan in their first major legislative test.

Division within their Republican Party means there may not be enough votes to secure the new bill.

Conservative Republicans have condemned the bill as too similar to the law it is meant to replace. Democrats and moderate Republicans, meanwhile, fear it will take insurance away from millions of people.

"No deal," Mark Meadows, chairman of the Freedom Caucus, a group of conservative members of the House, said.

Meadows and his group of more than two dozen Republicans met Trump on Thursday to try to get more concessions to reduce requirements on insurance companies.

Politicians said a vote would be held on Friday, despite the unclear outcome.

READ MORE: Americans split over replacing Obamacare

Repealing and replacing former president Barack Obama's healthcare law was one of the major campaign promises of Trump, who ran as a master dealmaker.

Mark Peterson, a professor of public policy at the University of California, said that getting the bill passed is "extremely critical" for Trump's ability to move forward with his agenda.

"Usually when a president comes in, this stage is what we call a honeymoon period. He's trying to rack up some big wins in Congress to build momentum," he told Al Jazeera.

"Right now his White House has been described as in a bit of chaos. He has not been doing well in popular support. To have this vote put off and perhaps even have a defeat would be quite a setback."

Conservative Republicans oppose the new legislation because it scraps Obamacare, but puts another government plan in its place. They believe healthcare should be left to the free market.

READ MORE: The Obamacare debate is not just about healthcare

Obama signed the Affordable Care Act, widely known as Obamacare, in 2010, providing health insurance coverage for 20 million low-income Americans previously uninsured. Many middle-income Americans complained their premiums spiked as a result.

Millions of Americans would lose coverage next year under the Republican plan, according to a review by the Congressional Budget Office made before last-minute amendments to the bill.

Supporters of Obamacare staged rallies in Washington, DC, Chicago and Los Angeles on Thursday denouncing efforts to repeal the law.

The Republican bill would halt Obama's tax penalties against people who do not buy coverage and cut the federal-state Medicaid programme for low earners, which the Obama statute had expanded.

It would provide tax credits to help people pay medical bills, though generally less than Obama's statute provides.

It would also allow insurers to charge older Americans more and repeal tax boosts that the law imposed on high-income people and health industry companies.

The measure would also block federal payments to Planned Parenthood for a year, another stumbling block for Republican moderates.