If 70% of the US rejects the job that a Dem controlled Congress is doing how badly will they lose in midterms?

No. Congress didn't have good approval ratings before the 2008 election either and Democrats gained many more seats. I don't think that people will vote for Republicans because Republicans basically have nothing of interest to say. Constant criticism of the other party does not make for viable suggestions as to how to run the government. Also, Republicans did a really poor job when they were in charge.