Temporarily (as we know most people in the states are conservative minded); that will be remedies in the normal if it happens, electoral process).. It will have the long term-effect of having much more conservative senators!
Let's look at the Senators who would not be in office if the 17th was repealed. Neither of the Senators from Alabama, Mississippi, North Carolina, Maine, one of the two from Tennessee, one of the two from Kentucky, David Vitter from Louisiana, either Coburn or Inhofe from Oklahoma. It's not just headcount. Losing senators from the South and Oklahoma would also tend to remove the more conservative ones.
I will also point out that the Democrats picked up several state legislatures in 2006. That doesn't bode well for your belief that the Democrat control would be temporary.