The American growth engine is starving for fuel. Political support for further government pump-priming has drained away. Consumers are tapped out. And exports will never come close to providing the needed economic boost.

Only corporations have the money to sustain today's fragile economic recovery. By year's end, American businesses will have amassed half a trillion dollars in savings.

Getting corporate leaders to act like capitalists, not Scrooges -- to invest rather than save -- is the nation's most pressing economic challenge. "The corporate reinvestment process has broken down," said Rob Parenteau, the head of a global financial advisory firm. "And when that happens, growth is short-circuited."

In 2009, corporate savings, less capital expenditures, totaled $300.8 billion. And this year, nonfinancial companies are on pace to save an additional $230.5 billion. That's the largest hoard of cash that U.S. corporations have accumulated in any two-year period since World War II.

The problem is "corporations don't see good investments out there," according to William Gale, co-director of the Tax Policy Center at the Brookings Institution. "They already have excess existing capacity, and they see relatively low demand for their stuff."

With inadequate growth in demand, businesses will fail to invest. Coupled with the reluctance of both government and consumers to spend, the result will be even weaker economic growth -- which will, in turn, lead to even less corporate investment.

The best way for the country to break this vicious cycle would be for Washington to continue to spend. But, with Congress almost sure to be more fiscally conservative after the November elections, further government spending does not seem to be in the cards.

The most straightforward approach would be to tax corporate cash holdings, forcing treasurers to spend their savings rather than give the money to Uncle Sam.

A better approach might be to use the tax code to encourage corporations to invest some of their retained earnings in rebuilding America's crumbling infrastructure, where some economists estimate the investment shortfall now exceeds $2 trillion.

"What if corporations received a tax credit if they devoted a portion of their reserves to purchasing Build America Bonds," suggested Sherle Schwenninger, who directs the New America Foundation's economic growth program. The bond program, created by last year's stimulus package, subsidizes bond issuance for state and local governments. It has supported $116 billion in spending to date, but it is about to expire.

Sustaining the program with new sources of capital would help bolster the economy in an investment-friendly manner.

It is time to recognize that excess corporate savings have become the roadblock to economic recovery. Washington should offer business leaders a choice: They can watch government tax their savings away or they can invest in much-needed infrastructure.

"Money Q&A" and "Company Town" are featured exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.