I think the American public has had enough of illegal immigration. Many citizens are out of work. There is little prospects for good jobs opening up. This is the time to run on that issue.
As much as I might like to see impeachment of upper level Obama Administration officials, I do not think that is the issue to run on. I could very well be wrong, but my take on it is that these impeachment(s) would not be as popular with everyone as ending illegal immigration would be.
One may be required to accomplish the other...