Ezra Klein:

The immigration status quo is a worst-of-all-worlds for workers, as illegal immigrants exist outside the wage standards and labor laws that help level the playing field and avoid a high-speed race to the bottom. When these workers' rights can be enforced, they will no longer be so cheap relative to native-born workers, and thus native born workers may see their other advantages -- linguistic, cultural, etc -- win the day. If you really want to screw the low-income workforce though, keep the situation static, and force them to continually compete against a workforce that's paid less than minimum wage and isn't subject to the safety standards and sundry other regulations of legal workers.