From their press release [1]:

The American Civil Liberties Union today [5/25/07] expressed grave concerns about the due process and privacy implications of the Senate immigration bill. The proposed legislation would create a vast federal database to verify the work eligibility of all job applicants in America - including U.S. citizens; expand indefinite detention; and deny effective judicial review of Department of Homeland Security errors denying immigration status.

...The proposed legislation would require every job applicant in America to have their eligibility to work verified by the DHS, using the error-plagued Employment Eligibility Verification System (EEVS). EEVS creates a massive government database containing extraordinary amounts of personal information on everyone in America, tied to each individual's Social Security number. If DHS makes a mistake in determining work eligibility, there will be virtually no way to challenge the error or recover lost wages due to the bill's prohibitions on judicial review.

As a part of EEVS, every person in America would be forced to carry a hardened Social Security card perhaps containing biometric information about the cardholder - essentially a national ID - and present a Real ID-compliant driver's license to get any new job. The proposed legislation also expands current practice of expedited removal. The ACLU noted that these policies do nothing to solve the problems of illegal immigration and violate the fundamental American value of due process.

In addition to their many othe flaws, the ACLU is indirectly linked to the Mexican government. However, that doesn't stop me from agreeing with them in principle, despite the fact that I think they're overselling the downsides. And, the danger in this case is that the bill could be amended in a way that would make them - and other far-left and racial power groups such as La Raza - support it.

[1] aclu.org/immigrants/gen/29878prs20070525.html