The lack of the right to a fair trial can be a bar to extradition, in general. But the bar is high: the "destruction of the very essence" of the right to a fair trial (described further below), which seems quite far from a trial which is not itself in doubt, but where some point of law might be appealed to a different court, some of whose members were suspected of bias in a different case.
While the specifics of extradition law are quite different between European countries, there are some overarching principles from the European Convention on Human Rights which apply to just such a circumstance.
The main relevant doctrine is from Soering v UK (application no. 14038/88, 7 July 1989), which established the basis for the Convention rights being applicable in extradition. Ordinarily, the Convention's protection doesn't extend to happenings in non-signatory countries such as the United States; but in Soering it was decided that
where substantial grounds have been shown for believing that the person concerned, if extradited, faces a real risk of being subjected to torture or to inhuman or degrading treatment or punishment in the requesting country
the responsibility of the sending state is engaged (para 91). That case was about a foreseeable Article 3 violation regarding the infliction of the death penalty in the U.S. Subsequent case law has extended to alleged violations of Articles 5 (right to liberty) and 6 (right to a fair trial), for various destination countries.
So in principle, one can challenge ones extradition on the basis of a potential Article 6 violation. But there are several nuances for how such allegations might be proved, for the United States or any other country.
- The destination country is not held to ECHR standards itself. For contracting states, there is a specific legal history concerning (say) the meaning of an "impartial tribunal" and rules for judicial recusal. Rather, the test is about whether there could be a "flagrant denial" of the right (Soering para 113), something amounting to "a breach of the principles of
fair trial guaranteed by art 6 which is so fundamental as to
amount to a nullification, or destruction of the very essence,
of the right guaranteed by that article" (Othman v UK (application no. 8139/09, 17 January 2012) at para 260).
- The test in relation to extradition to face trial is whether there are "substantial grounds" that the applicant "would be exposed to a real risk of being subjected to a flagrant denial of justice" (Othman para 261). Some different considerations might arise in relation to "risk" if the trial has already taken place, as the language of "risk" isn't always apposite for things which have happened.
- In making such an argument, an applicant will often want to say things about the past conduct of the destination country, e.g. that they have violated human rights on several occasions. If so then the court must be convinced "beyond a reasonable doubt" that such violations have taken place, but this term in ECtHR jurisprudence does not necessarily mean the same thing as it does in English or US law. It encompasses "the coexistence of sufficiently strong, clear and concordant inferences or of similar unrebutted presumptions of fact" (Jalloh v Germany (application no. 54810/00, 11 July 2006) at para 67). The destination state has an opportunity to try to rebut such allegations, or give assurances that they do not impinge on the case at hand.
- There is only very limited scope for departing from the test and standard of proof just described. The only admitted exception concerns evidence obtained through torture, where the standard of proof that torture has taken place is lower (and this was at issue in Othman). The UK Supreme Court recently rejected an argument that judicial bias might be in the same category; Popoviciu v Curtea De Apel Bucharest [2023] UKSC 39 at 73, citing Othman para 265 for "breaches based simply on defects in the trial process or in the composition of the trial court" not being as fundamentally wrong as torture.
Accordingly, allegations concerning the corruption, bias, or incompetence of U.S. judges could certainly be raised, but one would have to convince the court that there actually were "substantial grounds" for a "real risk" of a "flagrant denial of justice" - one that amounted to "destruction of the very essence" of a fair trial.
Allegations of judicial bias were part of Popoviciu (that the judge in Romania had a friendly relationship with one of the prosecution witnesses) and in that case the UK Supreme Court ended up saying that a court considering extradition would have to take into account not just the alleged conduct of the judge, but the whole system in the destination country regarding remedies for judicial bias (para 108). Along the same lines, alleged corruption of a U.S. Supreme Court Justice may not add up to a "real risk" if there are enough other safeguards in the system to mitigate the risk - including the low probability of anybody's case ending up before that court at all. (But, as an aside, the Presidential pardon power likely doesn't count as one of the safeguards, following Reyes v The Queen [2002] UKPC 11 at 44.)
Developing circumstances in the United States, or any country, would affect a court's assessment of the factors mentioned above. For example, if there is a long and independently assessed track record of human rights violations as in point (3) above, then a court would naturally be easier to convince that future similar violations might recur. Prison conditions in the United States have often been discussed in that context. Likewise, the U.S. generally promises that it will not seek to impose the death penalty after extradition and conviction, and so far it's kept that promise: if it ever breaks that commitment, then courts can take note. A recent example was the Julian Assange extradition, where the High Court (in London) accepted U.S. assurances that Assange would not be incarcerated at ADX Florence, Colorado, mainly on the basis of such promises always having been upheld in the past.
National governments may also react in their own way to a change in the human rights climate of another country. For example, the UK has an extradition treaty with the USA, implemented in domestic law via the specification of the USA as a "Category 2 Territory" for the Extradition Act 2003. The government could change that by secondary legislation (on the domestic plane) and denounce the treaty (on the international plane). Such an action would have international political consequences, but would also have the effect of halting extradition in both directions.
