One bedrock principle of U.S. constitutional law is that "the Constitution is different." Different principles and considerations apply when construing the Constitution than when construing statutes, regulations, or other enactments. One of the doctrines that have developed in response to Chief Justice Marshall's oft-quoted dictum that "we must never forget that it is a Constitution we are expounding," McCulloch v. Maryland, 17 U.S. 316, 407 (1819), is the constitutional avoidance doctrine.
Simply put, the constitutional avoidance doctrine requires that courts always try to resolve a case on nonconstitutional (i.e. statutory or other) grounds, and only interpret the Constitution when no other option remains. This doctrine was first enunciated in U.S. ex rel. Attorney General v. Delaware & Hudson Co., in which the Court held that:
It is elementary when the constitutionality of a statute is assailed, if the statute be reasonably susceptible of two interpretations, by one of which it would be unconstitutional and by the other valid, it is our plain duty to adopt that construction which will save the statute from constitutional infirmity."
213 U.S. 366, 407 (1909).
For example, let's say that Congress passes a statute that provides that "[n]o one shall, directly or indirectly, provide material support for terrorism by producing, publishing, or otherwise disseminating propaganda which tends to favour a terrorist organisation, its aims, or ideology1." Clearly, with a statute such as this, it is quite easy to go straight to the First Amendment to resolve the issue, most likely by invalidating the statute altogether. However, the constitutional avoidance doctrine requires that we first determine whether there is any halfway reasonable way of reading the statute that would render it constitutional.
It is worth noting that "avoiding a constitutional issue" is generally seen as license to do all sorts of things with the statutory language that are not normally covered by warranty. Since the interest in avoiding interpretation of the Constitution is so overriding, courts will often go to great lengths to avoid issuing opinions that may fundamentally change the constitutional landscape. One common artifice is that of "implication." In order to save a statute from the constitutional hatchet, the court will "imply" certain requirements into the statute, usually reasoning that Congress presumably did not intend to violate the Constitution in enacting the statute.
There is one major exception to the constitutional avoidance doctrine: State statutes. While the federal courts (particularly the Supreme Court) have the last word on the meaning of Acts of Congress, the federal judiciary does not have the authority to construe State statutes. A State's highest court's interpretation of a State statute is considered authoritative and unreviewable by federal courts; thus, a federal court may only uphold or invalidate a constitutionally challenged State statute.
Let's return to our semi-hypothetical statute:
No one shall, directly or indirectly, provide material support for terrorism by producing, publishing, or otherwise disseminating propaganda which tends to favour a terrorist organisation, its aims, or ideology.
Looking at the plain language of the statute, it seems rather hard to save. By its terms, it would appear to prohibit everything from recruiting suicide bombers (directly providing material support for terrorism by disseminating propaganda which favours a terrorist organisation) to writing a scholarly article arguing that Israel's occupation of the West Bank and Gaza violates international law (indirectly providing material support for terrorism by publishing "propaganda" which tends to favour the ideology of a terrorist organisation). If a court adopts this construction of the statute, there is no end to the constitutional problems it raises: due process violations2 as well as all manner of First Amendment problems.
A court seeking to avoid deciding whether the statute is constitutional will approach this dilemma by examining the challenged provisions of the statute. Here, for example, the provisions most likely to be constitutionally challenged would include the lack of a scienter (knowledge) requirement, the lack of any specified form of criminal intent, the "directly or indirectly" language, and the criminalisation of "propaganda" that "tends to favour" the "ideology" of a terrorist organisation3. Dodging the first two constitutional issues is relatively simple. We can assume that Congress did not intend to violate the Due Process Clause by failing to provide fair notice and punishing people who unwittingly say or write something that somehow favours a terrorist organisation; thus, scienter and intent will be implied. The statute, as construed, will now look something like this:
No one shall knowingly, directly or indirectly, provide material support for terrorism by producing, publishing, or otherwise disseminating propaganda which tends with intent to favour a terrorist organisation, its aims, or ideology.
However, this still leaves significant First Amendment free speech problems. Given the unambiguously unconstitutional language, which punishes all sorts of constitutionally protected "core" speech, most courts would be inclined to invalidate the statute on First Amendment grounds. However, assuming that a court was inclined to avoid striking the statute down altogether, it would have to engage in some pretty impressive acrobatics. Essentially, the statute would have to be read to "imply" application of the standard articulated by the U.S. Supreme Court's leading "seditious advocacy" case, Brandenburg v. Ohio, 395 US 444 (1969), which allows for punishment of seditious advocacy only when it incites to "imminent lawless action," Id. at 449. Thus, the statute will ultimately emerge from the meat grinder looking like this:
No one shall knowingly, directly or indirectly, provide material support for terrorism by producing, publishing, or otherwise disseminating propaganda with intent to favour a terrorist organisation, its aims, or ideology in such a way as to incite to immediate violence or breach of the peace.
Surveying the analysis necessary to "avoid a constitutional issue," one irony cannot be overlooked. In order to effectively "avoid" construing the Constitution, it is of course necessary to construe the Constitution as applied to the challenged statute; otherwise, there would be no way of knowing whether there was any constitutional issue to avoid, let alone how to construe the statute in order to avoid it. Thus, constitutional "avoidance" will generally cause the very evil it is supposed to circumvent. When applying constitutional avoidance, a court necessarily will reach a holding (or at the very least some very suggestive dicta) about what precisely the Constitution requires of a particular statute. Indeed, the constitutional avoidance approach is often far more specific than simply striking down an unconstitutional statute. While invalidating a statute will entail explaining why the statute was unconstitutional (interpretation of the Constitution), "avoiding" the constitutional issue requires a full-fledged rewrite of the statute, explaining not only why there are constitutional problems with the statute but how to sidestep them in the future.
1This statute is only partially hypothetical, as the current "material support" statute is arguably broad enough to encompass "ideological support."
2The statute, so construed, would require punishment of people who had no notice that their conduct would fall within the statute; nor does the statute require that propaganda be disseminated with the knowledge or intent that a terrorist organisation benefit from it.
3Of course, there are myriad other constitutional problems with the statute; listing and examining them all, however, would likely overload most web browsers. | 
