For more than 18 months, a rewrite of laws governing federal criminal punishments has been touted as the exception that was going to prove the rule: An effort that had so galvanized both conservatives and liberals that it would become one of the few memorable policy achievements of the current Congress.  

Well, the rule has held true about the deadlocked-by-polarization Capitol becoming only more so in the sessions before a presidential election. But the exception, by fits and starts, is growing ever less likely to be exceptional.  

“Sentencing reform,” as it’s known on the Hill, is seriously stuck.  

On the surface, it may not appear that way. Just offstage, there’s a fundamental impasse that looks as if it can only be broken if one sitde caves in, thereby imperiling the highly unusual bipartisan coalition that has been the issue’s signature feature.  

Complicating matters further, there are solid presidential and congressional campaign rationales for a deal, but also political arguments in opposition being at least as forcefully expressed.  

All this is on clearest display in the Senate, where the legislation looks to be riding a little wave of momentum but may be close to publicly coming off the rails – buffeted by anxieties about Willie Horton on the right and anger at Wall Street greed on the left.  

Two weeks ago, an ideologically diverse group announced revisions to the legislation they described as almost guaranteeing passage with a filibuster-proof majority of more than 60 and most senators from both caucuses.  

Its extraordinary roster of co-sponsors , now 19 Democrats and 16 Republicans, ranges ideologically from unvarnished liberal Barbara A. Mikulski of Maryland to libertarian tea party conservative Mike Lee of Utah. It includes both the whips, Republican John Cornyn of Texas and Democrat Richard J. Durbin of Illinois, along with a handful from each side getting mentioned in early vice-presidential speculation.  

Maybe most telling, the only Senate Democrat in any re-election trouble this fall, Michael Bennet of Colorado, is a cosponsor and so are four of the six Republican incumbents in competitive campaigns : Richard M. Burr of North Carolina, Roy Blunt of Missouri, Rob Portman of Ohio and Mark S. Kirk of Illinois.  

The bill would ease the mandatory minimum sentencing guidelines for future nonviolent offenders (mostly drug criminals), allow many of those already incarcerated out of prison ahead of schedule and ease their path back into society and the workforce.  

This exceptionally diverse group has decided that package is something they can stand behind as good social policy, law enforcement policy and budget policy. (Fewer federal inmates mean reduced prison expenses.)  Members in tough races are also eager to distance themselves from the “Do Nothing Congress” label by getting their fingerprints on an understandable and tangible accomplishment.  

That’s why there’s a decent chance the bill will come to the floor this summer, assuming the appropriations process inevitably seizes up and there no longer is the need to devote the Senate’s time to spending bills.  

Along the way, the measure is going to face one assault from powerful Republicans determined to kill it outright, and another from Republicans willing to love it to death.  

Ted Cruz of Texas, who returned to the Capitol this week vowing to press ahead with the combative outsider tone of his presidential campaign, and Jeff Sessions of Alabama, the first senator to endorse de facto GOP nominee Donald Trump, are leading the lambasting of the bill as going way too soft on crime.  

A floor debate would give Cruz an opportunity to put his scorched-earth style for opposing legislation back on C-SPAN display. And though Trump has not taken an explicit position on the bill, his many authoritarian statements suggest he’ll take Sessions’ advice and come out emphatically against it – especially if his likely opponent, Hillary Clinton, who’s become newly critical of “mass incarceration,” decides to endorse the bill.  

So it’s quite easy to envision law-and-order groups producing 30-second TV spots, evocative of the legendary Willie Horton ad from the 1988 presidential campaign, chiding even the GOP backers of the bill as pro-drug-dealer criminal justice weaklings.  

The other big obstacle, which might prove even more problematic, goes by the much nerdier label, mens rea.  

That Latin phrase, which translates as “guilty mind,” is law school shorthand for the way prosecutors are sometimes required to prove a defendant’s criminal intent in order to obtain a conviction. Under federal law, many categories of behavior are crimes only when the accused know what they’re doing is wrong and do it anyway – but some actions can bring convictions and imprisonment whether or not there’s any willful criminal intent.  

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Related: Criminal; Justice Debate Turns to How Much Is Possible

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Many influential Republicans, urged on by their business allies and such conservative fundraising forces as the Koch brothers, are eager to apply a blanket mens rea requirement across the federal criminal code. They say the government has too much power to convict companies and their executives without having to prove any criminal intent. And they are eyeing the sentencing overhaul bill as their best available vehicle for getting the job done.  

Lawmakers and activists from the Bernie Sanders wing of the Democratic Party deride this proposal as a thinly veiled effort to deliver a permission slip for more “What, me worry?” sketchy behavior to the same sort of bad actors in the corporate and investment worlds who melted down the economy eight years ago. These liberal forces, too, have the ability to produce punchy campaign commercials targeting those in Congress who go along.  

Even if the bill gets through the Senate without having to swallow the mens rea poison pill, top Republicans in the House are insisting that sentencing legislation will only move if it’s lashed together with their efforts to expand the need to prove criminal intent. The Obama administration argues the opposite, that the only way to sign a bill on sentencing this year is to negotiate protections for unwitting white collar criminals on a separate track.  

One again this campaign season, it’s the small clusters of combative voices at the edges that are likely to have more power than any collaborative majority in the middle.  

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