Sen. Bernie Sanders (I-VT) on Friday urged Americans considering voting for third-party presidential candidates to instead vote for Hillary Clinton, painting Donald Trump as a risk that the country cannot afford.

“Let us elect Hillary Clinton as president, and the day after, let us mobilize millions of people around the progressive agenda,” Sanders said in an appearance on MSNBC’s “Morning Joe.”

The Vermont senator, who has endorsed Clinton and is expected to campaign for her in Ohio this weekend, said that he understood voters’ frustration with the two-party system. But he warned that “protest” votes could contribute to a Trump victory.

Asked about a New York Times/CBS News poll released Thursday showing that a third of voters under the age of 30 plan to vote for third-party candidates, Sanders offered his “pitch” to those younger voters, who overwhelmingly supported him in the Democratic primary.

“Look, I was a third-party candidate. I began my career running as a third party, getting 2 percent and then 1 percent,” Sanders said. “I’m the longest serving independent in the history of the U.S. Congress, but I think that before you cast a protest vote, because Clinton or Trump will be president, think hard about it. This is not a governor’s race. It’s not a state legislative race. This is the presidency of the United States.”

“And I would say to those people out there who are thinking of the protest vote, think about what the country looks like and whether you’re comfortable with four years of a Trump presidency,” he continued.