There doesn't have to be a "harsh" crackdown. The border patrol is pathetically understaffed right now. Simply increasing the number of patrollers to the equivalent number of American soldiers currently patrolling Bosnia's borders would sharply reduce the number of illegals who cross over - and probably catch a number of terrorists as well - without having to be harsh about it at all.
In fact, the present understaffed situation increases the risk of violent incidents which would give fodder for the liberals against the administration.
It doesn't even have to be strictly a border patrol issue, the illegals can be stopped by fining the businesses that employ them and making it unprofitible for them, by tracking down the illegals after they enter and deporting them - which would send a strong message, and by making it impossible for illegals to get drivers licenses, educational benefits and welfare programs. But it all has to begin with somebody at the top giving a hoot. Immigration is good, unchekced illegal immigration is self destructive.