McCain and Hillary have consistently opposed any border enforcement crackdowns.
Hilary's reading the public much better on this than President Bush is. It goes without saying that she's not really going to do anything except "talk the talk", but that could be enough. Even lefty friends of mine are openly concerned about the borders and, since she'd have their support in any case, she'll talk up border security to peel off other votes from anybody naive enough to believe that she'd actually follow through.
It's amazing that McCain believes he can win the conservative support in the 2008 primaries with his liberal record. Maybe in the Northeast but I doubt anywhere else.