In some cases, "due process" or "equal protection" issues may come into play. Many places have laws which haven't been enforced for many decades because technological or sociological changes have rendered them obsolete. Despite (or perhaps because of) the lack of enforcement, however, nobody has bothered to remove such laws from the books.
If someone were charged with violating such a law, the fact that the law had not been enforced for ages, and that nobody made any particular effort to comply with it, could be a basis for acquittal, especially if there was any reasonable possiblity that the prosecution of a particular case was motivated by any illegitimate factors.
The fact that a law hasn't been enforced is no bar to prosecution if the reason for the lack of enforcement is that the law has been well-enough respected that enforcement has not been necessary. In such a case, the lack of enforcement would not favor the defense but might, if anything, favor the prosecution.
More generally, a law that society perceives as not existing may be rendered unenforceable by such perception. A law that is well-ingrained into society may not.
Also remember that we are talking about the same people who tried to argue that Roe v. Wade was a "super duper precedent" because of the number of times that it came before the Supreme Court and was not overturned. I think they all just make up these arguments as they go, looking for what's best for them in each separate situation.
-PJ
Certainly the equal protection principles in the due process and equal protection clauses preclude selective enforcement. This does not mean government cannot prosecute just because it did not do so in the past. It simply means that it must do so evenhandedly.
I'm not sure I'd go so far as to say an "obsolete" law can never be enforced. Suppose there is a law that says you can't send obscene telegrams, for example. Nobody sends telegrams anymore. However, if somebody did send an obscene telegram, that would not mean that there was a due process bar to prosecuting the statutory violation. When a law is on the books, that is constructive notice of the law's existence. Actual notice is not required.
Also, unless fundamental rights or a suspect classification are involved, the threshold for overcoming a due process/equal protection challenge is the extremely permissive "rational basis" test. All the government has to show is that enforcing the law in the particular case is rationally related to some legitimate government interest.
I got the impression from your post that you perceived this rational basis test to be met only if the reason for previous lax enforcement was that in the past there were not an inordinate number of violations. I respectfully disagree. Suppose there were significant violations and such violations were leading to increased social problems. It seems to me that curing these undesirable effects would be a legitimate government interest and again, enforcement of a previously ignored law would be a perfectly rational way of furthering that interest.
In short, the issue is not so much government's power to enforce the law "wearing off" as it is about the manner of enforcement: it must be evenhanded and serve a legitimate purpose.
None of these principles is offended in Jefferson's case. While it is true that congressional representatives are not always held accountable for taking bribes and many legislators probably think they have free reign to engage in such behavior because of lax enforcement, I can guarantee that the average reasonable person still believes bribery is illegal. Renewed enforcement (if it is indeed renewed) is justified by the legitimate interest in holding officials responsible for violating the public trust; prosecution is rationally related to that legitimate interest. Even Jefferson, Hastert, and company do not pretend legislators are immune from criminal prosecution for bribery.
For that reason, I think the case law you seem to be alluding to about selective/renewed enforcement does not speak to the issue here. That body of precedent dealt with when legal violations may be prosecuted, not the manner in which an investigation or subsequent prosecution is carried out.
That said, let's assume those cases do apply to the search of Jefferson's office. Here we had government agents acting pursuant to a warrant. Jefferson was treated just like millions of other Americans who are the target of criminal investigations. He cannot seriously invoke equal protection. In fact, he appears to be doing quite the opposite: he is engaging in special pleading, asking to be treated in a manner different from other citizens.
I suppose theoretically, Jefferson could say he is being unfairly singled out because he is a congressman, but legislators are not a suspect class and in any event the argument has little merit under the rational basis test. We have more, not less, of an interest in punishing corrupt leaders than in prosecuting the citizens who collude with them in their underhanded schemes. When ordinary citizens engage in corruption, the public does not perceive a failure in government; when leaders act this way, the public does have this perception.
Finally, there is no argument that Congress’ fifth amendment due process or equal protection rights are being violated. Quite simply, government entities are not protected by the Fifth Amendment. The Fifth Amendment reads: “No person shall … be deprived of life, liberty, or property without due process of law.” By its terms, it only purports to protect “persons.” Governmental entities like the legislature are not “persons.”