WASHINGTON—Republicans see a golden age of prosperity ahead, driven by the tax-and-spending megabill they are trying to push through Congress by July 4. Nonpartisan experts project far more modest effects, forecasting a slight near-term economic expansion and larger federal budget deficits.

The growth debate is at the core of this summer’s fiscal fight. Republicans are trying to focus public attention on growth—from tax cuts, deregulation and fossil-fuel production—and play down the Congressional Budget Office estimate that the bill would increase budget deficits by $2.4 trillion through 2034. The White House highlights growth to bolster congressional support, countering claims from Elon Musk and others that the package irresponsibly darkens America’s fiscal picture .

Republicans and outside economists agree on the basic direction: tax cuts increase consumer spending and business investment, accelerating short-term growth. But they differ vastly on how large and meaningful that jump would be. The bill, according to public- and private-sector economists, would fall far short of Republicans’ hoped-for boom.

“We would expect some dynamic revenue, some revenue feedback in that larger economy,” said Garrett Watson, director of policy analysis at the Tax Foundation, which favors lower tax rates and a simpler system. “But it wouldn’t come close to paying for itself.”

President Trump said in a social-media post last month that the U.S. annual growth rate would triple or even quintuple the 1.8% in CBO’s January forecast, which doesn’t incorporate the effects of any GOP policies. Since 2005, real U.S. gross domestic product growth hit or exceeded 3% twice: in 2018 after the 2017 tax cuts, and in 2021 during the recovery from the pandemic. House Republicans assume a 2.6% growth rate, yielding enough revenue to cover the megabill’s deficits.

“The economy is going to explode in capital formation. Jobs will increase. Wages will increase,” Senate Finance Committee Chairman Mike Crapo (R., Idaho) said after meeting with Trump last week. “We’re going to see the kind of growth and strength that this country wants.”

‘I would call this flat’

Broadly, economists across the political spectrum discount elected officials’ predictions.

Tax Foundation : The conservative-leaning group estimates that the bill would boost long-term GDP by 0.8%, generating enough revenue to cover about one-third of its costs. That is compared with doing nothing and letting tax cuts expire Dec. 31. The gain is like adding an average of 0.1 percentage point to the annual growth rate; reaching 3% would require much larger changes, Watson said.

Penn Wharton : Its budget model projects a 0.4% increase in GDP over the first decade. That is equivalent to raising the annual growth rate to 1.85% from 1.8%. “Basically, I would call this flat,” said Kent Smetters, who runs the Penn model. “We all know this is all going to get swamped by all the randomness.”

Joint Committee on Taxation : The nonpartisan congressional scorekeeper projected that the bill’s tax components would produce short-run growth through increased labor supply and capital stock. That would be counteracted by rising budget deficits, with a net effect of taking 1.83% annual growth to 1.86%. JCT estimates that the bill’s tax provisions would cover less than 3% of their costs with revenue from economic growth.

Yale Budget Lab : The think tank says the bill would bump the growth rate roughly to 2% from 1.8% through 2027, before the drag of federal debt weakens and reverses that effect.

Those all contrast with the view of the White House’s Council of Economic Advisers, which has a far rosier scenario . It projects a 4.2% to 5.2% increase in short-term GDP and a long-term gain of 2.9% to 3.5%. That gain would be three to four times the Tax Foundation estimate, which itself is larger than Penn Wharton, Yale or JCT.

Tax cuts vs. tariffs

Economists caution that tax policy can’t move the needle much in the U.S. economy, particularly given higher costs and uncertainty caused by tariffs.

Still, putting money in taxpayers’ pockets could increase demand for goods and services. Lower business taxes—especially faster write-offs for equipment and factories—encourage investment and have the biggest bang for the buck .

Council of Economic Advisers Chairman Stephen Miran said growth after 2017 demonstrates that the Republican formula can work. The economy and incomes grew solidly in 2018 and 2019 before the Covid-19 pandemic scrambled everything.

“When Americans elected President Trump, they did so knowing that he was a pro-growth president,” Miran said. “The bill is going to create a vibrant, dynamic economy.”

Miran added that federal taxes as a share of GDP was barely unchanged from fiscal 2017 to fiscal 2024. According to CBO, revenue was 17.3% of GDP in 2017 and 17.1% in 2024.

“There was no long-term hole in revenues,” Miran said.

But before the tax cuts passed, CBO forecast revenue increasing to 18.3% in 2024, and the law changed that trajectory. One of the most thorough academic studies found that the 2017 law increased domestic business investment but didn’t come close to paying for itself.

Not as strong as in 2017

The Tax Foundation’s Watson said policymakers should expect a more muted response from extending the 2017 tax cuts than from creating them. The bill includes new and revived business incentives but schedules them to expire.

“It’s pro-growth,” Watson said. “The more you add in some of these gimmicks and temporary changes, the more watered-down it gets.”

Senators including James Lankford (R., Okla.) and Steve Daines (R., Mont.) are seeking changes to encourage growth. They are particularly focused on making permanent some business-tax provisions such as immediate deductions for equipment purchases.

“If you have an expiration, you just don’t get predictability,” Lankford said.

Capital-investment incentives would be muted because tariff uncertainty complicates business planning, said Seth Carpenter, global chief economist at Morgan Stanley, which estimates that the bill would boost growth in 2026 before turning neutral and then negative.

Some projects might make sense with high tariffs but not lower ones. Even with the bill’s new deduction for factory expenses, without tariff certainty, Carpenter said, “I don’t think you’re going to be in any sort of hurry to start breaking ground.”

Kimberly Clausing, a former Biden administration economist now at the University of California, Los Angeles, said she worries about the drag from budget deficits if the GOP plan becomes law.

“If they failed,” she said, “I actually think that would be the best possible macroeconomic outcome.”

Write to Richard Rubin at richard.rubin@wsj.com