Add another hardened conservative to the front against harsh mandatory-minimum prison sentences: Texas Republican Ted Cruz.

The Senate Judiciary Committee voted 13-5 yesterday to approve a bill that would, among other things, slash mandatory-minimum sentences for federal drug crimes in half and curb the runaway growth of the U.S. Bureau of Prisons.

Cruz signed on as a sponsor of the bill at the last minute yesterday. It’s an important boost to the conservative contingent lining up behind sentencing reform, whose highest-profile members so far have been Utah’s Mike Lee and Kentucky’s Rand Paul. Arizona Republican Jeff Flake also voted for the measure yesterday.

To be sure, the crime warriors of the 1980s got their licks in, too. Iowa Senator Chuck Grassley got three new mandatory minimums attached to the bill before it moved to a final vote. But the momentum is clearly on the side of the left-right alliance of lawmakers who back sentencing reform, validating President Barack Obama’s approach of keeping a low profile on the issue.

That policy remains wise, even if it may disappoint liberal critics. As Steve Teles and I have argued, the most important development in crime politics over the last decade is that conservatives are developing their own, authentic ideology opposing mass incarceration. Cultivating this sentiment is critical, because conservative support will be indispensable if we are to make any deep dents in our 2-million strong prison population. Support from the likes of Mike Lee and Ted Cruz is the best insurance against Grassley-style tougher-than-thou posturing.

Rolling back mandatory minimums will still be a tough slog. Prosecutors have made their displeasure known: An association representing U.S. Attorneys put out a letter opposing the bill yesterday, and New York Senator Chuck Schumer said that district attorneys in his state were also complaining.

It’s worth noting that these prosecutors do not rest their case for these mandatory minimum sentences on the notion that they are fundamentally just. Their arguments stress equally that these sentences are an important investigative tool – a point of leverage the authorities can use to “flip” minor players in drug organizations to testify against the higher-ups.

That highlights an important development in American criminal-justice. To the traditional four justifications for punishment – rehabilitation, retribution, deterrence, and incapacitation – we have effectively added a fifth: coercion of testimony. That may be appropriate, but it’s not something we have explicitly acknowledged or thoughtfully debated.

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David Dagan

David Dagan is a freelance journalist and a PhD student in political science at Johns Hopkins University. Find him on Twitter: @DavidDagan