Rob Portman's speech at the Republican National Convention on Wednesday night was overshadowed by those of Paul Ryan and Condoleeza Rice, but it may have been the most substantive of the bunch.

"China manipulates its currency giving it an unfair trade advantage. So why doesn’t the president do something about it?" Portman asked. "I’ll tell you one reason – President Obama could not run up his record trillion dollar deficits if the Chinese did not buy our bonds to finance them…This will end under Mitt Romney."

One problem: China has stopped manipulating its currency. Joe Gagnon, an international finance expert at the Peterson Institute, explains that China hasn't added to its reserves of other countries' currencies in over a year now. Of course, that doesn't preclude manipulation in the future - but, for the time being, it's not happening.

"I think there is a serious risk that they will start up again, but all the Chinese I have spoken with this year believe they will never do that again," Gagnon explained. "We'll see." But for the moment, Japan, Singapore and Switzerland are more actively manipulating their currencies.

That makes the claim that we need the Chinese to buy our bonds to keep interest rates on U.S. debt low highly dubious. What’s more, Gagnon explains, even if all manipulators stop buying US bonds, the Fed can step in and buy them to keep US interest rates unchanged. Deficit-financed spending would be no more difficult than it was before.

But let's suppose manipulation starts back up again in January 2013. Is Obama or Romney better positioned to tackle it? It's hard to say, but Gagnon faults Romney for not offering many specifics for how he'd "get tough" on China. Obama, by contrast, has a pretty good record on the issue. "China’s manipulation really took off and peaked under George Bush," he explained. "Under Obama it has gradually disappeared."