President Obama just signed a new highway law, but the program runs for only two years, so either Obama or Mitt Romney is going to have to sign another one.

What should that law accomplish?

A trio of political leaders is asking the Commission on Presidential Debates to make infrastructure one of the six topics covered in the Obama-Romney debate in Denver on October 3.

"The overriding theme of this presidential election has been and continues to be the U.S. economy," write New York Mayor Michael Bloomberg, former Pennsylvania Governor Edward Rendell and former California Governor Arnold Schwarzenegger in an August 8 letter to the commission.

"Efficient, modern and reliable infrastructure is a necessary component to continued economic growth and smarter long-term investments that have the added benefit of creating jobs," they wrote.

"Unfortunately, the federal government has not continued the level of investment necessary to maintain and modernize our infrastructure to meet the growing population and demands of our society."

Bloomberg, Rendell and Schwarzenegger are co-chairs of Building America's Future, a bipartisan coalition of state and local officials lobbying for more infrastructure investment.

Both Obama and Romney support more investment. That's the easy part. The money is harder.

In his State of the Union Address last January, Obama called for spending half of the savings from winding down the Iraq war on infrastructure. He has resisted calls to boost the Highway Trust Fund revenue stream by raising fuel taxes, citing the impact such an increase would have on a fragile economy.

Romney, speaking in New Hampshire last December, said he's willing to borrow for infrastructure investment as long as there's a revenue stream to pay off the loan. He cited tolls and bonding as possible sources.

The Denver debate will focus on domestic policy. The topics will be chosen by the moderator, Jim Lehrer, executive editor of the PBS NewsHour.