In the United States, is belief in jury nullification generally a valid reason to challenge a jury for cause?
Based on these two sources:
my understanding is that in an official sense there's nothing that's considered an invalid reason to challenge a juror for cause. In other words, whatever it is that the attorney feels makes the juror unsuitable, he can take it to the judge, and the judge will rule based on what he thinks.
Yes; in fact, these challenges are made, and happen, routinely.
The phrase "jury nullification" is most often used by the extreme political right or extreme libertarians. But in fact all it means is a juror refusing to apply a law based on considerations other than the law he's instructed in by the judge.
In those jurisdictions that still have capital punishment, jurors are routinely asked whether their attitudes towards capital punishment would prevent them from applying it if it was justified under the law. If the juror says no, the juror is excluded for cause. The resulting jury is known as a "death-qualified jury."
In other words: a juror who honestly answers that he or she is unwilling to put aside personal, nonlegal considerations, but will instead vote his or her conscience, based on his or her belief that the law is unjust, is excluded from the jury. That's pretty much the definition of jury nullification, and the Supreme Court has said on multiple occasions that it's a good enough reason to keep someone off a jury.
This amusing site on the subject of jury nullification notes that, during voir dire, lawyers
will ask about nullification, usually in the slightly roundabout way: "Do you have any beliefs that might prevent you from making a decision based strictly on the law?"