Republican lawmakers in the Senate on Tuesday included a repeal of the Obamacare individual mandate in an amended version of its tax reform legislation.

"Today I am announcing my intention to amend the Senate tax bill to repeal the individual mandate and provide bigger tax cuts for middle income taxpayers," Senator Rand Paul said in a statement. "The mandate repeal is a promise we all made and we should keep."

Paul said doing away with the mandate would save the government more than $300 billion in the next decade as fewer Americans sign up for government-subsidized health insurance knows as Medicaid.

The Affordable Healthcare Act (ACA), often called "Obamacare", requires Americans to purchase health insurance, often at a subsidized rate if their annual income falls below a certain threshold, or pay a fine. The mandate has been heavily criticized by Republicans since the ACA's passage in 2010, and many have promised to repeal the measure now that President Donald Trump is in the White House.

The Senate's Committee on Finance is currently reviewing the chamber's proposed tax legislation, which is being considered in concert with a similar tax overhaul bill in the House of Representatives that does not repeal the individual mandate, as of yet. All Republicans on the committee supported the repeal.

"We're going to repeal the tax on poor Americans," John Cornyn, a committee member, told reporters Tuesday.

Trump tweeted support of the repeal late Monday, calling the mandate "unfair" and "highly unpopular".

But some moderate Republicans have already voiced apprehension about Paul's amendment.

"I personally think that it complicates tax reform to put the repeal of the individual mandate in there," said Senator Susan Collins.

The nonpartisan Congressional Budget Office said last week repealing the mandate would save $338 billion over the next 10 years, but it would also result in 13 million more Americans without health insurance.

Critics say that the mandate is crucial because it requires healthier people to buy insurance, lowering premiums for everyone.

"Tens of millions of middle class families will pay higher taxes under the GOP tax scam, and now Republicans want to force them to pay higher health premiums too," said House Minority Leader Nancy Pelosi in a statement. "Republicans' addiction to repealing the Affordable Care Act and raising Americans' health costs makes a terrible tax bill downright deadly."