According to this article (America's hidden agenda in Syria's war) the US is pressuring Syria's rebels to turn against Al-Nusra and fight it.
The writer compares the Al-Nusra with the Taliban and claims that the US missed a chance to talk with the Taliban - something done now by Karzai - because it considered them the same as Al Qaeda and didn't recognize the differences. The same would now apply to Al Nusra. I am not so sure. It is well known that the popularity of the Taliban in Afghanistan is below 10% so they can expect little from elections. So it is unlikely that they will compromise unless they think it improbable that they can achieve better results with arms. The same applies to Al Nusra.