Sen. Bernie Sanders (I-V.t.) last week became the first presidential candidate to call for allowing people convicted of crimes to vote while behind bars, the next front in the battle for voting rights.

“In my state, what we do is separate. You’re paying a price, you committed a crime, you’re in jail. That’s bad,” Sanders said at a Saturday town hall in Iowa. “But you’re still living in American society, and you have a right to vote. I believe in that, yes, I do.”

But no other candidates seeking the Democratic presidential nomination in 2020 have joined Sanders in endorsing the move on a national level since then. The idea is being pushed by the American Civil Liberties Union, which is urging candidates to commit to cutting the federal prison population in half and advancing civil libertarian ideas over the next two years.

It’s significant that candidates running for office are even talking publicly about restoring voting rights to people with felony convictions, said Marc Mauer, executive director of The Sentencing Project, a criminal justice nonprofit. Mauer said in an interview he couldn’t recall the issue ever coming up in a campaign before, and it was meaningful that it was being discussed as a legitimate policy question.

If voting is a guaranteed right, it shouldn’t be something that gets taken away in prison, Mauer added. He said it was inconsistent for candidates to describe voting as a fundamental right but support restrictions on voting in prison.

“Some people say when this comes up, ‘Voting is not a fundamental right; it’s a privilege.’ I guess they get to decide who gets to exercise that privilege. That seems to me a pretty slippery slope,” he said.

The furthest other Democrats were willing to go was to say they were open to considering whether people in prison should be able to vote.

Chris Harris, a spokesman for Sen. Kamala Harris (D-Calif.), said the 2020 contender believes “our first priority should be restoring voting rights to the millions of formerly incarcerated people who are still being denied access to the polls.” He added, however, that Harris “is open to other restoration of rights as well.”

Sen. Elizabeth Warren (D-Mass.), who is also running for the Democratic presidential nomination, stopped short of endorsing the idea in an interview on Capitol Hill. Warren supports restoring voting rights to people once they are released from prison.

“When people are in prison, their civil rights are suspended. Not all their civil rights are suspended. I think it’s an open conversation about what happens there,” Warren told HuffPost on Wednesday.

Former Rep. Beto O’Rourke (D-Texas) supports allowing someone to vote once they are out of prison, a spokesman told HuffPost. Sens. Cory Booker (D-N.J.), Kirsten Gillibrand (D-N.Y.), and Amy Klobuchar (D-Minn.) all support restoring voting rights to felons once they are released from prison.

“I’m definitely for felons’ rights. I just haven’t thought about that one,” Gillibrand told HuffPost on Wednesday.

Just two states ― Maine and Vermont ― currently allow people to vote while incarcerated for a felony conviction. Over 1.3 million of the 6.1 million people who couldn’t vote because of a felony conviction in 2016 were incarcerated, according to an estimate by The Sentencing Project. Several countries allow at least some prisoners to vote.

Klobuchar said she supported “what they did in Florida, which is when [felons] get out they get to vote.”

Floridians approved a state constitutional amendment last year that automatically restores voting rights to people with felony convictions once they complete their sentences, including probation and parole, a historic move expanding the right to vote to about 1.4 million people.

South Bend, Indiana, Mayor Pete Buttigieg also supports restoring voting rights “for all formerly incarcerated,” a spokesman said. “Just not while still incarcerated.”

A spokeswoman for Rep. Tulsi Gabbard (D-Hawaii) said felons should not be able to vote while under the control of law enforcement ― including if they’re on parole ― because their votes could be “unduly influenced by those authorities.”

Mauer said the reluctance to support voting while incarcerated may be shaped by fear.

“When it comes to prison, I think people haven’t thought it through much and they have this gut feeling that ‘Oh, no, we can’t do that because these people are prisoners.... These are bad people, and what would they do if they have the right to vote?’” he said in an interview.

“It’s hard to speculate whether they haven’t thought it through or their political advisers don’t want them to go that far, whatever the reason may be. Or that showing support for people post-sentence, they think is viewed in a positive light by many of their supporters.”