Former Democratic presidential candidate Bernie SandersBernard (Bernie) SandersBoogeywomen — GOP vilifies big-name female Dems RealClearPolitics editor: Moderate Democrats are losing even when they win Sanders tests his brand in Florida 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 Clinton‘Prosperity and peace’ is the winning Republican theme for midterms Mueller recommends Papadopoulos be sentenced to up to 6 months in prison Poll: Dem opponent leads Scott Walker by 5 points MORE over Republican nominee Donald TrumpDonald John TrumpWhite House counsel called Trump 'King Kong' behind his back: report Trump stays out of Arizona's ugly and costly GOP fight Trump claims he instructed White House counsel to cooperate with Mueller MORE

Libertarian presidential nominee Gary JohnsonGary Earl JohnsonThe Hill's Morning Report — GOP seeks to hold Trump’s gains in Midwest states Gary Johnson launches New Mexico Senate bid Gary Johnson eyeing Senate bid 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.