WASHINGTON >> The new GOP tax plan delivers a big tax cut to the wealthiest Americans while some in lower tax brackets would end up paying more, according to an analysis today from prominent nonpartisan researchers.

The plan being touted by President Donald Trump as the biggest tax cut ever delivers 50 percent of its total tax benefit to taxpayers in the top 1 percent, those with incomes above $730,000 a year, according to the Tax Policy Center of the Urban Institute and Brookings Institution. For those wealthy taxpayers, their after-tax incomes would increase 8.5 percent next year.

For other taxpayers, though, the benefits are far more modest or non-existent, the report finds. Taxpayers in the bottom 95 percent would see tax cuts averaging 1.2 percent of after-tax income or less next year.

And about 12 percent of taxpayers would face a tax increase next year, of $1,800 on average. That includes more than a third of taxpayers making between about $150,000 and $300,000, mostly because of the elimination of many itemized deductions.

By 2027, taxes would increase for about a quarter of Americans, including nearly 30 percent of those earning about $50,000 to $150,000 a year, and 60 percent of people making $150,000 to $300,000, according to the study.

“The number of taxpayers with a tax increase rises over time,” it said. That’s because the Republican plan would replace personal exemptions, which are tied to inflation, with some tax credits that aren’t tied to inflation.

The findings were certain to fuel the Democrats’ main attack line against the GOP plan: That it’s a giveaway to the rich at the expense of the middle class. Republicans immediately disputed the analysis.

“This so-called study is misleading, unfounded and biased,” said House Ways and Means Chairman Kevin Brady, R-Texas. “TPC makes a variety of overreaching and unrealistic assumptions about policy decisions members of Congress still have to make as we draft pro-growth tax legislation. Republicans are unified in delivering tax reform that will lower taxes on middle-class Americans, ensure they are able to keep more of their hard-earned money, and grow our economy.”

The Tax Policy Center noted that its analysis was preliminary, based on a proposal that itself lacked key information, such as the proposed income brackets that would correspond to the three new tax rates Republicans envision replacing the current seven.

The findings came as Senate Republicans unveiled a budget plan that lays the groundwork for their effort to overhaul the nation’s tax system. Provisions in the budget would allow Senate Republicans to pass the tax package with a simple majority of votes, preventing Democrats from being able to block the legislation and rendering Democratic votes unnecessary.

The Tax Policy Center’s analysis was based on an ambitious framework released Wednesday by the Trump administration and congressional Republicans that aims to reform the loophole-ridden code and dramatically cut corporate rates, from 35 percent to 20 percent. It’s the GOP’s marquee legislative project this year, following the embarrassing failure on health care.

Trump described the tax plan today as a “giant, beautiful, massive, the biggest ever in our country, tax cut.”

The tax legislation can advance only after House and Senate passage of the budget blueprint. The Senate Budget Committee intends to vote on its plan next week. A companion measure is headed for a House vote next week as well.

The new budget plan would permit the upcoming tax measure to add $1.5 trillion over the coming decade to the $20 trillion national debt. The Tax Policy Center finds the GOP tax plan would reduce federal revenues by $2.4 trillion over the next decade.

Without the budget passage, Senate Majority Leader Mitch McConnell of Kentucky said in a statement, “Democrats will continue to play partisan politics and obstruct our efforts to get our economy flourishing and growing at its full potential.”

More broadly, the Senate plan promises a balanced budget over the coming decade, but it relies on rosy projections of economic growth and spending cuts that Republicans have no plans to deliver. It would keep Pentagon spending mostly frozen at current levels, rather than the almost $90 billion increase demanded by GOP military hawks.

The budget also contains a provision that could allow the Senate to approve legislation opening up drilling in the Arctic National Wildlife Refuge. That’s a longtime goal for Republicans, including Alaska Sen. Lisa Murkowski, a moderate whose vote will be needed on tax legislation.