Sen. Cory Gardner, R-Colo., listed off on Twitter the types of people who would be voting under Bernie Sanders' idea to let all incarcerated felons vote.

"The Supermax prison in Florence, CO is called the 'Alcatraz of the Rockies' because it houses the worst criminals and terrorists in the country. Apparently some Democrats want to make it the next battleground precinct by giving these inmates the right to vote," Gardner tweeted.

He then listed well-known criminals housed in ADX Florence.

Sanders defended the idea during his town hall with CNN on Monday.

"If somebody commits a serious crime, sexual assault, murder, they're going to be punished," Sanders said. "They may be in jail for 10 years, 20 years, 50 years, their whole lives. That's what happens when you commit a serious crime. But I think the right to vote is inherent to our democracy. Yes, even for terrible people.

"Because once you start chipping away and you say, 'Well that guy committed a terrible crime, not going to let him vote,' or 'That person did that, not going to let that person vote,' you're running down a slippery slope," he said.