WASHINGTON — Closing in on the first major legislative achievement of his term, President Donald Trump on Saturday defended the Republican tax cut as a good deal for the middle class while boldly suggesting it could lead to explosive economic growth.

The legislation, which the GOP aims to muscle through Congress next week, would lower taxes on the richest Americans. Benefits for most other taxpayers would be smaller, but Trump attempted to sell the bill as a “Christmas present” for middle-class Americans in part because it would trigger job growth.

“It’ll be fantastic for the middle-income people and for jobs, most of all,” Trump told reporters on the White House lawn before traveling to Camp David for the weekend. “And I will say that because of what we’ve done with regulation and other things our economy is doing fantastically well, but it has another big step to go and it can’t take that step unless we do the tax bill.”

No stranger to hyperbole, Trump also predicted the legislation would cause the economy to soar beyond its current 3 percent rate of growth.

“I think we could go to 4, 5 or even 6 percent, ultimately,” the president said. “We are back. We are really going to start to rock.”

Many economists believe that attaining consistent 4 or 5 percent annual growth would be challenging. The nation last topped 5 percent growth in 1984.

The Republican plan is the widest-ranging reshaping of the tax code in three decades and is expected to add to the nation’s $20 trillion debt. The tax cuts are projected to add $1.46 trillion over a decade.

Under the bill, 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.

“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 Friday. “Most critics out there didn’t think it could happen. … And now we’re on the doorstep of something truly historic.”

The bill would repeal an important part of President Barack Obama’s Affordable Care Act — the requirement that all Americans have health insurance or face a penalty — as the GOP looks to unravel a law it failed to repeal and replace this past summer. It came together as Republicans cemented the needed support for the overhaul, securing endorsements from wavering senators.

Marco Rubio of Florida relented in his high-profile opposition after negotiators expanded the tax credit that parents can claim for their children. He said he would vote for the measure next week.

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.