WASHINGTON — After weeks of quarrels, qualms and then 11th-hour horse-trading, Republicans revealed their huge national tax rewrite late Friday — along with announcements of support that all but guarantee approval next week.

The legislation would slash tax rates for big business and lower levies on the richest Americans in a massive $1.5 trillion bill that the GOP plans to muscle through Congress before its year-end break. Benefits for most other taxpayers would be smaller.

“This is happening. Tax reform under Republican control of Washington is happening,” House Speaker Paul Ryan of Wisconsin told rank-and-file members in a conference call. “Most critics out there didn’t think it could happen. ... And now we’re on the doorstep of something truly historic.”

According to the 1,097-page bill released late Friday, today’s 35 percent rate on corporations would fall to 21 percent, the crown jewel of the measure for many Republicans. Trump and GOP leaders had set 20 percent as their goal, but added a point to free money for other tax cuts that won over wavering lawmakers in final talks.

The legislation represents the first major legislative achievement for the GOP after nearly a full year in control of Congress and the White House. It’s the widest-ranging reshaping of the tax code in three decades and is expected to add to the nation’s $20 trillion debt. The debt is expected to soar by at least $1 trillion more than it would without the tax measure, according to projections.

Support is now expected from all Senate Republicans, ensuring narrow approval. Democrats are expected to oppose the legislation unanimously.

“Under this bill, the working class, middle class and upper middle class get skewered while the rich and wealthy corporations make out like bandits,” said Senate Minority Leader Chuck Schumer of New York. “It is just the opposite of what America needs, and Republicans will rue the day they pass this.”

The bill would drop today’s 39.6 percent top rate on individuals to 37 percent. The standard deduction — used by around two-thirds of households — would be nearly doubled.

Those who itemize their taxes face mixed results. The $1,000 per child tax deduction would grow to $2,000, with up to $1,400 available in IRS refunds for families who owe little or no taxes.

But the deduction that millions use in connection with state and local income, property and sales taxes would be capped at $10,000. Deductions for medical expenses that lawmakers once considered eliminating would be retained.

Only on Friday did Republicans cement support for the major overhaul, securing endorsements from wavering senators.

Marco Rubio of Florida relented in his high-profile opposition after negotiators expanded the child tax credit, and he said he would vote for the measure next week.

Rubio had been holding out for a bigger child tax credit for low-income families. After he got it, he tweeted that the change was “a solid step toward broader reforms which are both Pro-Growth and Pro-Worker.”

The tax package would double the basic per-child tax credit from $1,000 to $2,000. The bill makes a smaller amount available to families even if they owe no income tax. That amount was increased to $1,400 from the $1,100 level of an earlier version of the bill.

Low-income taxpayers would receive the money in the form of a tax refund, which is why it’s called a “refundable” tax credit.

Sen. Bob Corker of Tennessee, the only Republican to vote against the Senate version earlier this month, made the surprise announcement that he would back the legislation. Corker, the chairman of the Senate Foreign Relations Committee, has repeatedly warned that the nation’s growing debt is the most serious threat to national security.