In Herrera, the petitioner advanced his claim of innocence to support a novel substantive constitutional claim, namely that the execution of an innocent person would violate the Eighth Amendment. 28 Under petitioner’s theory in Herrera, [ SCHLUP v. DELO, ___ U.S. ___ (1995) , 15] even if the proceedings that had resulted in his conviction and sentence were entirely fair and error-free, his innocence would render his execution a “constitutionally intolerable event.” Id., at ___ (slip op., at 1) (O’CONNOR, J., concurring).
Schlup’s claim of innocence, on the other hand, is procedural, rather than substantive. His constitutional claims are based not on his innocence, but rather on his contention that the ineffectiveness of his counsel, see Strickland v. Washington, 466 U.S. 668 (1984), and the withholding of evidence by the prosecution, see Brady v. Maryland, 373 U.S. 83 (1963), denied him the full panoply of protections afforded to criminal defendants by the Constitution. Schlup, however, faces procedural obstacles that he must overcome before a federal court may address the merits of those constitutional claims. Because Schlup has been unable to establish “cause and prejudice” sufficient to excuse his failure to present his evidence in support of his first federal petition, see McCleskey v. Zant, 499 U.S. 467, 493 -494 (1991), 29 Schlup may obtain review of his constitutional claims only if he falls within the “narrow class of cases . . . implicating a fundamental miscarriage of justice.” Id., at 494. Schlup’s claim of innocence is offered only to bring him within this “narrow class of cases.”
Schlup’s claim thus differs in at least two important ways from that presented in Herrera. First, Schlup’s claim of innocence does not by itself provide a basis for [ SCHLUP v. DELO, ___ U.S. ___ (1995) , 16] relief. Instead, his claim for relief depends critically on the validity of his Strickland and Brady claims. 30 Schlup’s claim of innocence is thus “not itself a constitutional claim, but instead a gateway through which a habeas petitioner must pass to have his otherwise barred constitutional claim considered on the merits.” Herrera, 506 U.S., at ___ (slip op., at 13); see also 11 F.3d, at 740. 31
More importantly, a court’s assumptions about the validity of the proceedings that resulted in conviction are fundamentally different in Schlup’s case than in Herrera’s. In Herrera, petitioner’s claim was evaluated on the assumption that the trial that resulted in his conviction had been error-free. In such a case, when a petitioner has been “tried before a jury of his peers, with the full panoply of protections that our Constitution affords criminal defendants,” 506 U.S., at ___ (slip op., at 2) (O’CONNOR, J., concurring), it is appropriate to apply an “`extraordinarily high’” standard of review. Id., at ___ (slip op., at 9) (O’CONNOR, J., concurring). 32 [ SCHLUP v. DELO, ___ U.S. ___ (1995) , 17]
Schlup, in contrast, accompanies his claim of innocence with an assertion of constitutional error at trial. For that reason, Schlup’s conviction may not be entitled to the same degree of respect as one, such as Herrera’s, that is the product of an error-free trial. Without any new evidence of innocence, even the existence of a concededly meritorious constitutional violation is not in itself sufficient to establish a miscarriage of justice that would allow a habeas court to reach the merits of a barred claim. However, if a petitioner such as Schlup presents evidence of innocence so strong that a court cannot have confidence in the outcome of the trial unless the court is also satisfied that the trial was free of non-harmless constitutional error, the petitioner should be allowed to pass through the gateway and argue the merits of his underlying claims.
Consequently, Schlup’s evidence of innocence need carry less of a burden. In Herrera (on the assumption that petitioner’s claim was, in principle, legally well founded), the evidence of innocence would have had to be strong enough to make his execution “constitutionally intolerable” even if his conviction was the product of a fair trial. For Schlup, the evidence must establish sufficient doubt about his guilt to justify the conclusion that his execution would be a miscarriage of justice unless his conviction was the product of a fair trial.