"Maybe this is personal, because I run for office," told MSNBC’s Rachel Maddow at the First in the South Presidential Candidates Forum at Rock Hill, S.C., on Friday.

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"It has never occurred to me as a candidate to figure out a way that I could deny the vote to people because they might vote against me," Sanders continued.

"What the Republicans are doing is so un-American, it is so outrageous, that it is literally beyond belief. They are political cowards, and if they can't face a free election, they should get another job."

Sanders said there was a "real crisis" in America and hinted at a constitutional amendment to declare that every American age 18 or older "is registered to vote, end of discussion."

Part of Sanders' strategy at Friday night's forum appeared aimed at closing Clinton's lead over him with African-American voters – a key demographic in South Carolina, where black voters make up more than half of the Democratic primary vote.

Eighty percent of African-American Democratic primary voters in South Carolina currently support Clinton, according to this month’s Winthrop Poll

Sanders wins the support of just 15 percent of black voters in the state.

The Palmetto State is the first southern primary and the first real test of which candidate will win African-American support.

If Sanders wins Iowa and New Hampshire – the first two Democratic Party nominating contests – Clinton will need to prove quickly that these states are aberrations due to their heavily white, liberal constituencies.