Updated at 10:30 am ET

President Trump and GOP congressional leaders have outlined their plan for the most sweeping overhaul of the federal tax code in more than three decades.

They're proposing deep cuts in both individual and corporate tax rates, saying that will help to supercharge a slow-growing economy, according to a document of the framework obtained by NPR's Tamara Keith.

"If we do this, we will create millions of new jobs for our people, and bring many, many businesses back to our shores," Trump said Tuesday, on the eve of the rollout. He described the proposed overhaul as "massive tax cuts that our country desperately needs to thrive, to grow, to prosper."

The president plans to promote the tax plan Wednesday afternoon at the Indiana State Fairgrounds in Indianapolis.

While many of the details still have to be filled in, House and Senate GOP leaders have been meeting for months with the Treasury secretary and a top White House economic adviser to fashion a plan they can agree on.

It would cut the top individual income tax rate from 39.6 percent to 35 percent and slash the corporate rate from 35 percent to 20 percent. The plan would also create a new, 25 percent rate for so-called "pass-through" businesses that are currently taxed at the individual rate.

"Tax reform is the most important thing we can do to restore confidence to this country, to get jobs and prosperity," House Speaker Paul Ryan, R-Wis., told reporters. "That is why we are so singularly focused on getting this done this year."

The plan would end taxes on the profits that multi-national companies earn outside the country, and allow those companies to repatriate past profits at a discounted rate. In addition, for at least five years, the plan would allow companies to write-off the cost of investments immediately, rather than deducting those costs over time as they do now.

Republican backers have framed the tax overhaul as a much-needed boost for the middle class. It would nearly double the standard deduction, so fewer people would have to go through the trouble of filing an itemized return.

"Many Americans are living paycheck to paycheck," said Rep. Cathy McMorris Rodgers, R-Wash., chair of the House Republican Conference. "This is our moment to lift the financial burden on these hard-working men and women, give people more money in their pocketbooks to help manage the cost of living."

Democrats, however, warned the GOP plan would provide a windfall for the wealthy. Only people earning more than $418,000 pay the top individual tax rate now. And congressional experts say most of the savings from a lower corporate rate would flow to investors, not ordinary workers.

What's more, the GOP plan would eliminate the estate tax — which applies only to those with assets of more than $5.49 million — as well as the Alternative Minimum Tax, which hits upper-income taxpayers.

"Democrats have strongly and firmly stood for the position that not one penny of tax cuts should go to the top one percent," Senate Minority Leader Chuck Schumer, D-N.Y., said in a statement. "If Republicans push a tax bill to please their hard-right wealthy contributors, instead of working from the middle, they will have the same trouble with taxes that they're having with health care."

As they tried to do with health care, Republicans are preparing to use a procedural move to avoid the threat of a Democratic filibuster in the Senate.

But Trump has also tried to win Democratic support for the tax plan. Democratic Sen. Joe Donnelly, who is up for re-election next year, is joining the president at the kickoff event in his home state of Indiana.

Trump has held similar, campaign-style rallies for the tax plan in Missouri and North Dakota — both states he carried last year with Democratic senators who will be on the ballot in 2018.

Backers say their plan will not shift the burden of taxes from those at the top of the income ladder to the middle or the bottom. They left the door open for a higher individual tax rate if necessary to achieve that goal.

Republicans hope to simplify the tax code by eliminating many deductions, although they've promised to preserve tax breaks for mortgage interest and charitable giving. The deduction for state and local taxes is on the chopping block. That would be especially costly for taxpayers in high-tax states like New York and California, many of which are governed by Democrats.

While closing tax loopholes would offset some of the lost revenue from lowering rates, the proposed tax plan could still balloon the federal deficit. Republican lawmakers reportedly plan to craft a budget that assumes up to $1.5 trillion in additional red ink over a decade.

Even that may not be enough to accommodate what proponents have described as "historic" tax cuts, likely totaling trillions of dollars. While some Republicans argue that faster economic growth sparked by the cuts would help to refill government coffers, independent experts are skeptical. Economists at Goldman Sachs estimated last week that tax cuts would add only 0.1 or 0.2 percent to GDP growth over the next couple of years. Both Treasury Secretary Steven Mnuchin and National Economic Council Director Gary Cohn are Goldman alums.

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