The democrats refused to ratify the KORUS agreement that George Bush negotiated. US auto manufacturers and the UAW opposed it, but supported Obama's renegotiated agreement. Also, the GOP retook the House of Representatives in the 2010.
The other big issue was US beef. The Korean tariffs on beef fall very gradually over a very long period.
Here's a couple of links if you want to be better informed.
Wikipedia Page on KORUS
Congressional Research Service report on KORUS(2014)