Republican presidential front-runner Donald Trump is running largely on the promise to bring back millions of Americans jobs lost to China and other foreign countries, but economists say that Trump stands no chance against the global economic forces that took the jobs away.

More than 5 million manufacturing jobs have left the U.S. since the beginning of 2000, with between 2 million and 2.4 million lost to competition from China, according to the Associated Press.

Around 12.3 million Americans are now employed in manufacturing, down from a peak of 19.6 million in 1979.

In announcing his candidacy on June 16, Trump said, "I'll bring our jobs from China, from Mexico, from Japan, from so many places. I'll bring back our jobs, and I'll bring back our money ... I will be the greatest jobs president that God ever created. I tell you that."

But according to Alan Blinder, Princeton University economist and former Federal Reserve Chairman, Trump's claims are "completely implausible."

Many of those low-skill jobs that went to Chinese workers earning $1 an hour are now being sent to even lower-wage countries such as Bangladesh and Vietnam.

If America tried to block foreign-made products and make everything at home, prices would soar and other countries could retaliate by blocking U.S. goods, explains AP.

Trump has repeatedly laid blame on past U.S. presidents for poorly negotiating trade deals, which led to an mass exodus of jobs from the U.S. manufacturing sector.

But Eswar Prasad, professor of trade policy at Cornell University, says that trade deals only play a modest role in job creation. "Better trade deals are unlikely to be a panacea," Prasad told AP.

Prasad said policymakers would have better luck investing in areas that can improve America's long-term competitiveness, such as schools, airports and roads.

And while many manufacturing jobs have gone overseas, even more have simply vanished and are unrecoverable because of technological innovations such as automation, reports The Washington Post.

For example, General Motors employed 600,000 in the 1970s, but now, it sells more cars than ever but only employs 216,000.

"No matter who becomes president, I cannot foresee a scenario where 5 million additional manufacturing jobs ... reappear in the U.S. in the decades ahead," economist David Autor of the Massachusetts Institute of Technology told AP.

With U.S. unemployment at a seven-year low of 5.3 percent, bringing all the outsourced jobs back to the U.S. could result in negative unemployment, according to Harold Sirkin, senior partner at the Boston Consulting Group and an expert on manufacturing competitiveness worldwide. "We'd have to bring in people from other countries to do the work," he said.