The economy wasn't in solid shape until the second half of '95. You also had the healthcare reform failure and the gov't shutdown. Then, on his watch, the Democrats lost control of both Houses of Congress. Most figured how in the hell is going to get anything done now?
But, Clinton moderated, the economy improved and he was very effective at letting others take the fall for some of his own scandals (i.e. Slick Willy).
Actually the economy was growing really fast and jobs increased dramatically from 1993Q1-1995Q2. Slowing down in 1995Q3-Q4. And going back up at a faster but moderate past.
I think people didn't felt the economy was improving by the 1994 midterms even though it was. But other reasons caused the loss. From the failed healthcare reform, passing gun control, and dont ask don't tell being implemented which was considered quite liberal back then. Which caused enthusiasm amongst conservatives both social and economic to use 1994 was a payback. Helped by the perot voters.