It is indeed plausible that these policies will encourage more capital investment in the next few years, said Joel Prakken, chief U.S. economist at Macroeconomic Advisers, with higher productivity growth as a consequence.

But that should be a one-time adjustment, after businesses increase their capital stock in response to more favorable policies. Once that process is complete and has resulted in higher productivity, there’s no reason to think that capital investment would keep rising as a share of the economy.

In other words, the benefits in terms of productivity growth and economic growth should fade over time, which the administration acknowledges.

“The new tax law would be a one-time shift, spread out over several years, after which there would be a new steady state growth path for labor productivity,” DJ Nordquist, chief of staff at the White House Council of Economic Advisers, said in an email. “Nevertheless, in that new steady state, faster growth than we have seen in recent years can still be expected because of the elimination of excessive regulations, to which this administration is committed and because of our infrastructure plan. This deregulation will have enduring benefits to the rate of growth.”

Some private economists are not persuaded these effects are powerful enough to account for continued strong growth a decade from now. “I try to fit these numbers into a mainstream paradigm and I can’t make them fit,” Mr. Prakken said.

What about other sources of growth?

But even if higher capital investment can’t do all the lifting of generating 3 percent growth, there remain those other two possibilities, of hours worked or total factor productivity, that could help achieve the 3 percent growth forecast even after the lift from tax cuts and capital investment fades.

But demographic trends are putting a lid on potential growth from more hours of work. The retirement of the baby boomers and stabilization of the proportion of women in the work force mean that potential hours worked will rise only 0.4 percent a year in the coming decade, according to the C.B.O.’s forecast (compared with 1.3 percent a year from 1950 to 2016).