Former Democratic presidential candidate Bernie SandersBernie SandersSanders apologizes to Biden for supporter's op-ed Jayapal: 'We will end up with another Trump' if the US doesn't elect progressive Former health insurance executive: Current system is bankrupting country MORE on Sunday discouraged voters from picking a third-party nominee, saying the issues facing the U.S. are too dire for a "protest vote." 

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"This moment in history for a presidential election is not the time for a protest vote," the Vermont senator said on CBS's "Face the Nation." 

"It's time to look at which candidate will work best for the middle class and working families." 

He said he didn't want to disparage third-party candidates, but he encouraged voters to pick Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonDemocratic debates are magnet for lobbyists NYT editorial board endorses Warren, Klobuchar for Democratic nomination for president Sanders v. Warren is just for insiders MORE over Republican nominee Donald TrumpDonald John TrumpSanders apologizes to Biden for supporter's op-ed Jayapal: 'We will end up with another Trump' if the US doesn't elect progressive Democrats: McConnell impeachment trial rules a 'cover up,' 'national disgrace' MORE

Libertarian presidential nominee Gary JohnsonGary Earl JohnsonThe 'Green' new deal that Tom Perez needs to make The Trump strategy: Dare the Democrats to win Trump challenger: 'All bets are off' if I win New Hampshire primary MORE has hoped to benefit from record low favorability numbers for Clinton and Trump. But a RealClearPolitics polling average only gives him 8.9 percent support nationally.