By Ben Leubsdorf

President-elect Donald Trump and his emerging team are confident their proposed tax cuts and regulatory rollbacks will spark a dramatic upswing in economic growth.

Two forces will make that tough: an aging population and stagnant productivity.

For years, Wall Street and government forecasters have waxed pessimistic on the economy’s capacity for sustained growth, pointing to multiple reasons it has diminished since the 1990s. Perhaps most important, demographic trends have produced slower growth in the working population. And the U.S. and other advanced economies are grappling with sluggish gains in productivity, for reasons that aren’t wholly clear.

As a result, the nonpartisan Congressional Budget Office predicts roughly 2% annual growth for gross domestic product pretty much as far as the eye can see.

But Mr. Trump says his tax, trade and other policy proposals will generate average annual GDP growth of 3.5% or more—a pace the U.S. hasn’t seen in more than a decade. The last stretch of sustained growth above that level ended in 2000.

“My economic plan rejects the cynicism that says our labor force will keep declining, that our jobs will keep leaving and that our economy can never grow as it did once before,” Mr. Trump said in September.

His pick for Treasury secretary, Steven Mnuchin, reiterated that target last week. “Our most important priority is sustained economic growth, and I think we can absolutely get to sustained 3% to 4% GDP,” he told CNBC.

Economists agree that tax cuts and infrastructure spending could give the U.S. a boost, especially in the short term. But doubt runs deep on both the left and the right that Mr. Trump can hit his ambitious target for long-run growth.

With the right package of policy changes “you can have a measurable impact on GDP growth,” said Douglas Holtz-Eakin, president of the conservative American Action Forum think tank and a former CBO director. “Can you get a full percentage point? That’s a lot.”

Jared Bernstein, senior fellow at the liberal Center on Budget and Policy Priorities, shares that skepticism. “I wouldn’t say it’s impossible, because it’s economics and not science,” he said. “But it’s as close to that as I’m comfortable asserting.”

The recovery that began in mid-2009 has been long by historical standards but also weak, with annual growth averaging just 2.1%. There are many forces that have been blamed for that tepidness, from the household-debt overhang after the housing bubble to Obama administration regulations.

But over the long run, an economy’s potential growth rate all comes down to how much people are working, and how productive they are. Deceleration in labor-force growth as the baby-boom generation begins to retire and workforce participation declines, plus a marked slowdown in productivity gains since the IT-fueled boom of the late 1990s and early 2000s, help explain the tepid recent growth trend and have prompted professional forecasters to lower their expectations for growth.

To be sure, economic forecasts are often wrong, and growth in the short run can exceed or undershoot what economists consider its long-run potential. During the current expansion, the quarterly growth rate for U.S. output has bounced around a range from negative 1.5% to positive 5%.

Fiscal and monetary policies that push growth beyond its potential level for too long, however, carry the risk of overheating the economy and generating uncomfortably high inflation. Achieving consistently stronger GDP growth would require some combination of expanding the workforce and boosting productivity growth—daunting tasks, especially on the demographic front.

Since Election Day, Federal Reserve officials have signaled they hope the new administration will pursue productivity-focused policies. Chairwoman Janet Yellen advised Congress in mid-November to “choose policies that would improve that long-run growth and productivity outlook.”

Mr. Trump has proposed overhauling the tax code, promoting domestic energy production, rebuilding the nation’s infrastructure, and reducing both the burden of government regulations and the trade deficit. His campaign described as conservative its estimate that his policies would mean average GDP growth of 3.5% a year.

Growth of 4% or even 5% a year is achievable “when we get the policies right,” said Stephen Moore, a conservative economist and economic adviser to Mr. Trump. He said tax and regulatory reforms and increased energy production can bolster business investment and draw workers back into the labor force.

“There’s so much low-hanging fruit out there,” he said.

Others have their doubts. In the short run, J.P. Morgan Chase forecasts a modest boost to GDP growth in the second half of 2017 and 2018 from fiscal stimulus. But Michael Feroli, the bank’s chief U.S. economist, said he would be “very surprised” if trend growth approached Mr. Trump’s target.

While there’s some scope for policy changes to boost long-run growth, “I think we’re talking in the range of a few tenths of a percent,” Mr. Feroli said.

Mr. Trump won’t likely be daunted by such skepticism. After all, Mr. Moore said, experts dismissed his chances of winning the presidency, too.