In 2016, the last year of President Obama's term, the Senate rejected what was seen as a major criminal-justice reform measure. The effort foundered on resistance from Sens. Tom Cotton and Jeff Sessions, who in 2017 resigned to become the Trump Administration's first Attorney General.
Key parts of that bill, including reduced sentences for drug offenses, were among provisions in the First Step Act, passed in 2018. Sen. Cotton continued to oppose the measure, but the combination of other bipartisan support, influential lobbying efforts and solid Trump Administration support for the bill overcame that opposition.