Vermont Sen. Bernie Sanders understands that some voters want to eschew Hillary Clinton and Donald Trump and instead cast their ballots for either Libertarian Gary Johnson or Green Party candidate Jill Stein in November—but on Sunday urged people to support Clinton, saying the stakes are too high this year for a “protest vote.”

“I think what the focus has got to be on now is understanding that this moment in history, for a presidential election, is not the time for a protest vote,” he said on CBS’ “Face the Nation.” “It is the time to look at which candidate is going to work best for the middle class and working families.”

Sanders, who first ran for Congress as an independent in 1990, said he has nothing against third-party candidates; in fact, he added, he thinks they’re important to American democracy. But Sanders noted that the stakes are particularly high in this campaign—and that the difference between Clinton and Republican nominee Donald Trump are incredibly stark. Polling has shown significant interest in voting for a third-party candidate this year, particularly among young people.

“Look, I am the longest-serving independent in the history of Congress. When I was younger I ran on a third party here in the state of Vermont,” he said. “So I’m not here to disparage third-party candidates who historically have played a very, very important role in this country in raising issues and moving this country in certain directions.”

He cited the various “crises” the country is facing, including “a disappearing middle class, massive levels of income and wealth inequality, the issue of the increase in bigotry we are seeing, climate change, the fact that so many young people are leaving school deeply in debt.”

As for Clinton’s standing with the millennial voters that flocked so much to Sanders’ own campaign during the Democratic primary, Sanders said if Clinton focuses on the issues she’ll do well with the key demographic. He and Massachusetts Sen. Elizabeth Warren have been campaigning on Clinton’s behalf in recent weeks, making her pitch to young voters.

“If she focuses on the issues she will do just really well with the American people and certainly with younger people,” he said.

One issue he cited in particular where the difference between Clinton and Trump is especially salient is climate change.

“Young people are very concerned, appropriately so, about the crisis regarding climate change,” he said. “Clinton has a pretty strong program which says we have got to transform our energy system away from fossil fuel. You know what Donald Trump’s position is on climate change? He thinks it’s a hoax, and that is really frightening for the future of this planet.”