Republicans have passed the largest tax overhaul in 30 years through the Senate, bringing them one step closer to giving President Donald Trump his first major legislative triumph after months of false starts.

A final burst of negotiations with some Republican hold-outs, and the addition of a number of amendments on Friday (local time) saw the bill pass 51-49 overnight.

The tax plan will: Slash the corporate rate from 35 per cent to 20 per cent

Slash the corporate rate from 35 per cent to 20 per cent Create three individual tax brackets with rates of 12 per cent, 25 per cent and 35 per cent

Create three individual tax brackets with rates of 12 per cent, 25 per cent and 35 per cent Allow the majority of Americans to file taxes on a single sheet of paper

Allow the majority of Americans to file taxes on a single sheet of paper Roughly double the standard deduction, a set amount of income exempt from taxation, for all taxpayers

Here's how the bill strikes a blow to Obamacare.

What's the Obamacare repeal?

The US Senate's tax overhaul strikes a huge blow against former president Barack Obama's healthcare law by repealing the requirement that all Americans obtain health insurance.

The individual mandate is meant to ensure a viable health insurance market by forcing younger and healthier Americans to buy coverage to help offset the cost of sicker patients.

It helps uphold the most popular provision of the law, which requires insurers charge sick and healthy people the same rates.

Why are experts arguing it shouldn't be removed?

Policy experts say the problem with removing it, while also keeping the rest of Mr Obama's Affordable Care Act intact, is that it is expected to cause insurance premiums to rise and lead to millions of people losing coverage.

"It's going to take a bunch of healthy people out of the insurance market," the director of the healthcare program at Northwestern University's Kellogg School of Management, Craig Garthwaite, said.

"[Obamacare] is going to collapse even more now."

Without the mandate, health insurance premiums would rise 10 per cent in most years over the next decade on the individual market and 13 million people would lose coverage by 2027, the nonpartisan Congressional Budget Office said in a report last month.

Kaiser Family Foundation health economist Larry Levitt said that insurers would need around $US10 billion per year to offset the lost revenue from the individual mandate rather than raise premiums.

As a result, insurers and leading medical groups urged Congress to preserve the individual mandate and warned of "serious consequences" if it were repealed.

So why did the Republicans include it?

What is Obamacare? Affordable Care Act passed by Congress in 2010

Affordable Care Act passed by Congress in 2010 Promised to help tens of millions of uninsured Americans get health coverage

Promised to help tens of millions of uninsured Americans get health coverage Under the plan, people can buy cheap insurance on healthcare.gov

Under the plan, people can buy cheap insurance on healthcare.gov Most coverage costs less than $US100 per month

Most coverage costs less than $US100 per month Policies vary according to person's income, location, family size and level of coverage desired

Policies vary according to person's income, location, family size and level of coverage desired More than 10 million people now have medical cover under the laws

More than 10 million people now have medical cover under the laws Number of uninsured adults reduced by 26 per cent

It's set to save the Government quite a bit of money, approximately $US300 billion, which they are using to offset some of the costs of the tax cuts for big corporations.

But more broadly, Mr Trump and Republicans in Congress had promised to repeal and replace Obamacare during the 2016 election campaign, having long argued it is a costly intrusion into the healthcare system.

They've spent the better part of seven years arguing against the law, which has made it harder for insurers to refuse to cover sick people.

They failed several times this year to scrap the mandate as part of a broader repeal of Obamacare, with more moderate Republicans concerned about providing health coverage to low-income Americans.

Mr Trump signed an Executive Order to make it easier for Americans to buy bare-bone health insurance plans. ( Reuters: Kevin Lamarque )

It goes further than an executive order signed in October, which made it easier for Americans to buy bare-bones health insurance plans, and critically, for small business employers to buy cheaper plans for their employees.

On that issue, they had been blocked by some of their own party's senators, including Susan Collins of Maine.

Is it set in stone?

There have been some options put forward to try and save it.

The final details of the tax bill still need to be hammered out with Republicans in the House of Representatives, who are expected to combine the bills as quickly as possible at a conference this week.

One of the proposed stabilisation bills, which has been co-authored by Republican senator Lamar Alexander and Democratic senator Patty Murray, would restore billions of dollars in subsidies that health insurers use to reduce out-of-pocket costs for low income Americans.

A second option, co-authored by Collins and Democratic senator Bill Nelson, would create an additional $US4.5 billion fund to compensate insurers for covering health care for the sickest patients.

But health policy experts said both of those measures would not make up for expected premium increases and the rise in the numbers of uninsured Americans.

"Neither of these bills would do anything to offset the increase in uninsured resulting from a mandate repeal," Mr Levitt said.

"The marketplaces would limp along without a mandate but it's probably not a stable place."

What else was added to the tax overhaul?

It was a bit of a last-minute scramble to get the tax bill over the line on Friday night, with Democrats complaining of handwritten amendments being made to the final draft just hours before the vote.

It is estimated that the last-minute amendments added nearly $US300 billion to the tax bill's costs.

Further tax deductions

In a bid to win over Wisconsin's Ron Johnson and Steve Daines of Montana, a change was made to the bill to allow deductions of 23 per cent, up from 17.4 per cent, for companies whose owners pay individual, not corporate, taxes on their profits.

Another change came from Senator Collins, who demanded that people would be allowed to deduct up to $US10,000 in property taxes, matching a House provision to keep some Republican votes from high-tax states like New York, New Jersey and California.

To pay for that, leaders decided not to erase the alternative minimum tax. Instead, they reduced the number of high-earners who must pay it.

They also increased a one-time tax on profits US-based corporations are holding overseas and they would require firms to keep paying the business version of the alternative minimum tax.

Promise to restore protections for Dreamers

While this wasn't added to the bill, it's worth noting that senator Jeff Flake ended up voting in favour for the bill after he "received commitments" from party leaders and the administration "to work with [him]" to restore protections for Dreamers, young immigrants brought to the US illegally as children. It had previously been dismantled by Mr Trump.

Drilling in the Arctic National Wildlife Refuge

Another provision allowed the Arctic National Wildlife Refuge to be opened up for oil and gas exploration.

According to the proposal, oil and gas drilling will be allowed on approximately 1.5 million acre coastal plain.

ABC/wires