Conservatives who remain dubious over immigration reform often want to separate the issues, emphasizing border security first, with consideration of legalization later. But the political problem is obvious: Democrats won't agree to reform without a path to legalization, any more than conservatives would agree without enhanced security.
In policy terms, the most effective way to discourage illegal immigration is to make it impossible for unauthorized entrants to get jobs - requiring tougher workplace enforcement and e-verify. But if that means millions who are currently working suddenly leave the workforce, the results would devastate the economy.
We should make it impossible for illegals to hold jobs, but we then need some way the undocumented can earn some sort of legal status to continue working. Separating out any one element of balanced reform would only make the situation worse.