By NICHOLAS RICCARDI, Associated Press

Democratic presidential contenders are facing a new debate over whether criminals in prison, even notorious ones like the Boston Marathon bomber, should be able to win back their right to vote.

Vermont Sen. Bernie Sanders says that they should and that voting is "inherent to our democracy — yes, even for terrible people." Many of his rivals for the 2020 nomination aren't as sure, and at least one opposes the idea outright. Sanders himself acknowledged that he was essentially writing an attack ad for Republicans to use against him through his support for the issue.

The question illustrates how Sanders continues to stand to the left of the other candidates as he endorses giving all prisoners, including those convicted of heinous crimes, the right to vote. Prodded by criminal justice activists, Democrats have largely embraced the politically safer cause of winning back access to the ballot box for felons who have served their time.

"Sen. Sanders is taking it one step farther than most campaigns have," said Mark Mauer of The Sentencing Project, a group that advocates letting prisoners vote. "Even if you're sitting in a maximum-security prison, you retain most of the fundamental rights of a U.S. citizen."

Only two states now allow prisoners to vote — Maine and Vermont, Sanders' home state. But debate over the issues has begun percolating in a handful of other states, backed by advocates who see it as a civil rights issue. Asked Monday during a CNN town hall whether he supported allowing sex offenders or terrorists such as marathon bomber Dzhokhar Tsarnaev to vote, Sanders did not hesitate.

"The right to vote is inherent to our democracy — yes, even for terrible people," Sanders said. "Once you start chipping away and you say, 'Well, that guy committed a terrible crime, not going to let him vote. Oh, that person did that, not going to let that person vote.' You're running down a slippery slope."

Sanders reiterated his support for the voting rights of prisoners on Wednesday, tweeting: "More than 30 countries around the world today such as Canada, South Africa and Finland allow prisoners to vote. This is not a radical idea."

At Monday's town hall, California Sen. Kamala Harris, a former prosecutor, stressed that she backed restoring voting rights to released convicts. Of those in prison, she said, "I think we should have that conversation."

Harris hedged further on Tuesday. "There has to be serious consequences for the most extreme types of crimes," she told reporters at a campaign stop.

Massachusetts Sen. Elizabeth Warren said she supports a constitutional right to vote but is "not there yet" on prisoners voting. Mayor Pete Buttigieg of South Bend, Indiana, opposes giving prisoners access to the ballot box. "Part of the punishment when you're convicted of a crime and you're incarcerated is you lose certain rights," he said during the town hall.

On Wednesday, former Rep. Beto O'Rourke and former Secretary of Housing and Urban Development Julian Castro both said they backed letting nonviolent felons cast ballots behind bars, but they drew the line at violent prisoners.

"For violent criminals, it's much harder for me to reach that conclusion," O'Rourke told reporters. "I feel like, at that point, you have broken a bond and a compact with your fellow Americans. There has to be a consequence in civil life for that as well."

Republicans were quick to criticize Democrats for even contemplating the idea. The Republican National Committee chairwoman, Ronna McDaniel, tweeted that Democrats were trying to "out-Bernie" each other. "Now they're refusing to rule out letting terrorists to vote from prison," she said.

The movement to restore voting rights to convicted felons has had some success in recent years.

Activists won their biggest victory in November when Florida voters overwhelmingly passed a ballot measure reversing the state's lifetime voting ban for most convicted felons who have completed their sentences. Advocates of a criminal justice overhaul have been able to chip away at restrictions through state legislatures in conservative states such as Kentucky and Alabama.

Chiraag Bains, a Justice Department official in the Obama administration who now works at the liberal think tank Demos, said restoring prisoners' right to vote is the logical next step in a movement that is trying to reverse laws enacted shortly after slaves won the right to vote. Within 15 years of the end of the Civil War, nearly one-third of the states passed laws barring felons and prisoners from voting, and some were explicit about trying to limit the new political power of African Americans, Bains said.

Restoring the right to vote "is long overdue," Bains said, noting the bipartisan support for measures loosening restrictions on released felons voting across the South. "I think the public is a lot smarter about criminal justice reform and voting rights" than they're given credit for.

But beyond Maine and Vermont, no state has gone so far as to loosen restrictions on those serving time behind bars.

The Sentencing Project estimates about 1.3 million citizens cannot vote because they are in prison or jail. Even liberal states such as California bar felons on parole or in prison from voting.

Last year, the activist group Initiate Justice tried to gather signatures for a ballot measure in California allowing all felons to vote, but it didn't have enough money to fund a campaign. Now it's trying to persuade the state's Democratic-controlled Legislature to put a measure on the 2020 ballot — but it would restore voting rights only to parolees, not those in prison.

The group's founder, Taina Vargas-Edmond, isn't sure the slimmed-down version will get through the California Legislature. "It has been something of a struggle," she said. "It's part of this whole tough-on-crime justice system that we're all wedded to."

Vargas-Edmond said it may be an easier political sale to win rights back for parolees, but she believes there's no need to bar those serving time, either.

"I doubt there's a single person who ever said, 'Well, let me not commit this crime because I'll lose my right to vote,'" she said.

The issue has begun bubbling up in Democratic-run legislatures with increasing frequency, a sign that prisoner voting rights are becoming a legitimate issue, activists say. "We're in a democratizing moment," Bains said.

But that doesn't mean Democrats are united on the issue.

Even in solidly Democratic Hawaii, a measure to let prisoners vote didn't make it out of the Legislature this year. In New Mexico, a bill to restore some felons' voting rights made incremental progress but only after a provision to let prisoners vote was removed. The amended bill still died in the Democratic-controlled Legislature.

Although he's a strong advocate of voting rights for those released from prison, state Sen. Bill O'Neill of New Mexico only signed on as a sponsor after the prison-voting clause was removed.

"That's too far for me," O'Neill said. "Practically, I'm just having a really hard time seeing that being implemented, and as a Democrat, I'm already concerned with the attack ads being printed up."

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Associated Press writers Hunter Woodall in Keene, N.H., and Meg Kinnard in Columbia, S.C., contributed to this report.