What is the test for a defendant to argue that a prior conviction was unconstitutionally obtained?

"[T]he Constitution requires that defendants be given the opportunity to collaterally attack prior convictions which will be used against them at sentencing." United States v. Vea-Gonzales, 999 F.2d 1326, 1333 (9th Cir.1993). The government bears the burden of showing the fact of a prior conviction, and then the burden shifts to the defendant to prove by a preponderance of the evidence that the prior conviction was unconstitutionally obtained. United States v. Newman, 912 F.2d 1119, 1122 (9th Cir.1990).