A proposed Pennsylvania law meant to deter would-be arsonists has become fodder for a debate over the fairness of the state's criminal-sentencing practices.

The bill seeks to lengthen prison terms for arsonists, in part by establishing mandatory minimum sentences. State Sen. John C. Rafferty Jr. (R., Chester) introduced the measure in March in response to Coatesville's wave of arson fires in 2009.

But the mandatory-minimum provision has drawn the opposition of the Senate Judiciary Committee chairman, who argues that such rules have caused prison overcrowding, led to injustices, and failed to deter crime.

Currently, judges have discretion in deciding the length of prison terms for arson, though they are supposed to consult guidelines developed by the state Commission on Sentencing. Judges may be bound to impose specific sentences if the arson also involves other crimes, like murder.

Rafferty's legislation would establish a new category of aggravated arson for people who set fire to structures with the intent of causing injury or who set fire to buildings with people inside them. It would also establish minimum sentences of between five and 10 years, depending on the fire's severity.

That's where it ran into trouble with committee Chairman Stewart J. Greenleaf (R., Montgomery), who has become a fierce critic of mandatory minimums. "There's always different factors that come into play [when someone commits a crime], and injustices invariably happen when we tell a judge that he has to impose a particular sentence," he said in an interview.

Greenleaf pointed to the "little fish" that get caught up in overly harsh sentences because of them.

He argued that the bill could end up leading to a teen who set a brush fire as a prank getting a 10-year prison sentence.

Though he conceded that opposing mandatory minimums for nonviolent offenses such as drug use is more popular than opposing them for violent crimes, Greenleaf vowed the bill would not come up for a vote in its current form.

And he's getting ready to introduce his own legislation that would allow judges to depart from mandatory minimums for nonviolent offenders when "a substantial manifest injustice would occur" with the sentence, he said.

Rafferty was miffed that a former longtime supporter of mandatory minimums has lined up against his bill. He said that it was carefully designed to be fair and argued that mandatory minimums play an important role in crime deterrence.

Richard Long, executive director of the Pennsylvania District Attorneys Association, also defended mandatory minimums, calling them an "important tool in ensuring justice."

Douglas A. Berman, an Ohio State University law professor who's written widely on mandatory minimums, said that district attorneys use them as a way of pressuring defendants into plea bargains and that they imply that "we don't trust judges to always impose a tough-enough sentence."

Five people were convicted or pleaded guilty for their involvement in the Coatesville fires.

Rafferty said that while he was pleased with the sentences the five received, the penalties could have been tougher with a mandatory minimum.

The sentences ranged from 242 days to 23 months for one person who pleaded guilty to setting two fires, to 121/2 to 25 years each for two people convicted of setting a number of fires.

In 2009, the last year for which data are available, a typical state prison sentence for the 104 people convicted of arson endangering another person - a comparable crime - was of 45 to 100 months.

Berman said mandatory minimums do not deter such crimes and Rafferty's bill would be more effective if it included specific sentencing guidelines for judges instead.

Mark Bergstrom, executive director of the Pennsylvania Commission on Sentencing, liked the guideline approach, saying it allows "a little more finesse" than simple mandatory sentences.

Rafferty and Greenleaf said they are in discussions about amending the bill.

Contact staff writer Anthony Campisi at 215-854-5015 or acampisi@phillynews.com.