Larry Summers echoes my interpretation of the electoral impact on trade policy:
The challenge for trade policy in the next two years will be more to resist protection than to further liberalisation, since both parties want to respond to the economically anxious middle class that disproportionately projects its anxieties on trade agreements. [FT]
On the other hand, Dan Griswold claims that the partisan shift matters:
The election means that the president’s trade agenda has come to a screeching halt. [AFP]
What? I must have missed Bush’s swiftly speeding trade agenda! The election is a tap on the brakes of a vehicle was coming to a rolling stop anyway.
Trade Diversion 
I think Dem v Rep matters for FTAs (labour and environment features) but not nearly as much for multilateral talks like Doha. The US trade policy is set by powerful special interest groups — so strong that Congressman and Senators cannot get elected unless they promise to pursue their constituents special interest — much like left or right, all French policy makers support French agric.