The federal system adopts procedural measures intended to make the Guidelines the lodestone of sentencing. A retrospective increase in the Guidelines range applicable to a defendant creates a sufficient risk of a higher sentence to constitute an ex post facto violation.Justice Kennedy did not join part III C of Justice Sotomayor's opinion, making that part a plurality opinion. That part concludes, "But, contrary to the dissent's view, see post, at 11-13, the Ex Post Facto Clause does not merely protect reliance interests. It also reflects principles of 'fundamental justice.' "
What makes this case close is that the Sentencing Guidelines were transformed from mandatory to advisory in the Booker case. Justice Thomas notes in the dissent (joined by Chief Justice Roberts and Justices Scalia and Alito):
First, the Guidelines do not constrain the discretion of district courts and, thus, have no legal effect on a defendant's sentence. Second, to the extent that the amended Guidelines create a risk that a defendant might receive a harsher punishment, that risk results from the Guidelines' persuasive force, not any legal effect. The Guidelines help district judges to impose sentences that comply with §3553(a). The risk of an increased sentence is, in essence, the risk of a more accurate sentence--i.e., a sentence more in line with the statutory scheme's penological goals. Guideline changes that help district courts achieve such pre-existing statutory sentencing goals do not create a risk of an increased sentence cognizable under the Ex Post Facto Clause. We have never held that government action violates the Ex Post Facto Clause when it merely influences the exercise of the sentencing judge's discretion.