Without a doubt, a national DNA database will lead to government abuse of the system no matter what. This database makes it easier for the government to track U.S. citizens, which is something the United States government enjoys doing. Abuse couldn't be avoided by any means, so such a system is ill-advised as far as libery is concerned.
A national DNA database in the United States would be a very blatant violation of citizens' Fourth Amendment rights - protection against search/seizure without probable cause/warrant- if participation was mandatory. Such a massive large-scale seizure of DNA from the American population would imply that there is probable cause to seize the DNA, which leads to the question - is every American guilty of some crime? Now, if it was completely voluntary, that would be OK, but does not in any way guarantee that the DNA database wouldn't be abused or the security of it wouldn't be breached in some way.