Former Democratic presidential candidate Bernie SandersBernard (Bernie) SandersCongress closer to forcing Trump’s hand on Saudi support Booker seeks dialogue about race as he kicks off 2020 campaign Capitalism: The known ideal 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 ClintonRoger Stone shares, quickly deletes Instagram photo of federal judge on his case Barack, Michelle Obama expected to refrain from endorsing in 2020 Dem primary: report Why the national emergency? A second term may be Trump’s only shield from an indictment MORE over Republican nominee Donald TrumpDonald John TrumpRosenstein expected to leave DOJ next month: reports Allies wary of Shanahan's assurances with looming presence of Trump States file lawsuit seeking to block Trump's national emergency declaration MORE

Libertarian presidential nominee Gary JohnsonGary Earl JohnsonPotential GOP primary challenger: Trump's 'contempt for the American people' behind possible bid The Hill's 12:30 Report — Presented by Kidney Care Partners — Trump escalates border fight with emergency declaration Former Mass. governor takes step toward Trump primary challenge 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.