US President Donald Trump has pulled the wraps off a huge new tax plan.

It was one of his key promises during the election campaign last year, and something he’s been promising to do all year.

Let’s take a look at it.

What’s in the plan?

Broadly, Mr Trump says he wants to simplify America’s tax code.

His proposal will:

• 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
• Put a surcharge on the very wealthy
• 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

Mr Trump said the plan was a “once-in-a-generation opportunity”.

“We want tax reform that is pro-growth, pro-jobs, pro-worker, pro-family and yes tax reform that is pro-American,” he said.

“Under our framework, we will dramatically cut the business tax rate so that American companies and American workers can beat our foreign competitors and start winning again.”

Who are the winners?

We don’t really know yet.

And it’s because a lot of the details of the plan still haven’t been released.

It’s pretty clear that American businesses, both big and small, would win out.

Wealthy Americans would definitely benefit from the cut in the top tax bracket, which would drop from 39.5 per cent to 35 per cent.

But exactly how the proposal would affect individual taxpayers is unclear because we don’t know the income levels at which the new proposed tax brackets would kick in.

What will this cost?

A LOT.

The Committee for a Responsible Federal Budget policy group estimated the plan contains about $US5.8 trillion over a decade of total tax cuts.

The trouble is, Republicans have only flagged $US3.6 trillion in offsetting revenue to pay for it, meaning it could add $US2.2 trillion to the US national debt.

In October, Republicans on the Senate Budget Committee agreed to allow a $US1.5 trillion ($1.92 trillion) debt cost on the plan, so expect a fight over which measure might get cut to help reach that target.

Plan prompts divided reactions

Republican House Speaker Paul Ryan said the plan was a “now or never moment”.

It also got a thumbs up form the US Chamber of Commerce business lobbying group and an organisation called the RATE Coalition, representing large American companies including AT&T Inc, FedEx Corp, Home Depot Inc, General Dynamics Corp and Walmart Stores Inc.

Democrats slammed the plan.

Here’s the top Senate Democrat Chuck Schumer:

“Under this plan, the wealthiest Americans and wealthiest corporations make out like bandits while middle-class Americans are left holding the bag.”

The top Democrat on the Senate Finance Committee, Senator Ron Wyden, explained the proposal like this: “If this framework is all about the middle class, then Trump Tower is middle-class housing.”

Senator Bernie Sanders, who ran against Hillary Clinton for the Democratic presidential nomination, didn’t mince words either.

“President Trump’s tax plan is morally repugnant and bad economic policy,” he said.