Who says you have to vote for John Kerry?
First, I didn't say that, but I'll answer your question anyway.
Since there are only two viable candidates, a voter is either helping Bush get elected or helping Kerry get elected. For example, in most states, a vote for Nader helps Bush.
You posited as to the issue of border control: George W. could lose the election because of this..
A Bush loss would mean a Kerry victory. So, I'll repeat the question I asked in response. "Do you seriously believe that Kerry would be any better?"
If you prefer, I can depersonalize it. "Does any one who would help Kerry defeat Bush seriously believe that Kerry would be any better?"