Sen. Bernie Sanders of Vermont speaks in support of Democratic presidential nominee Hillary Clinton at a rally at the University of Akron in Akron, Ohio, on Saturday. (Julie Carr Smyth/AP)

The crowd to see Bernie Sanders filled half of a college gym. Hundreds of millennials who still proudly wore their Bernie shirts listened to the senator from Vermont insist, as a nearby sign said, that he was “Berning for her.”

He’d always told them not to trust the media; now he was telling them not to listen when people attacked the Democratic nominee.

“Hillary Clinton and I believe that women should control their own bodies,” Sanders said. “Hillary Clinton and I believe that our gay brothers and sisters deserve the right to get married.”

It was not earth-shattering, but Democrats — to their surprise — thought it needed to be said. Worried by polls that show many voters under 30 choosing third-party candidates over Clinton, they’ve launched ad campaigns and surrogate efforts to pull those voters back.

Unlike similar campaigns to win over working-class white voters or conservative Democrats in the South, the millennial campaign is designed to push an open door. By every indicator, millennials are more progressive than their parents — pro-gay rights, pro-immigration reform, and even likely to say that they prefer socialism to capitalism.

Here's a look at what voters under 30 think about the 2016 presidential election, who they're leaning towards and how that compares to 2012. (Sarah Parnass,Osman Malik/The Washington Post)

“Do you believe in climate science or don’t you?” asked Sen. Tim Kaine (D-Va.), Clinton’s running mate, in a Sunday interview on “Meet the Press.” “Millennials do, Hillary Clinton and I do. Donald Trump doesn’t. Do you believe women should be able to make their own health-care decisions or don’t you? Millennials do, Hillary Clinton and I do, Donald Trump doesn’t.”

Those issues do not rank high among the priorities of older swing voters, but they’re stronger with millennials — and thanks in part to the Sanders challenge, Clinton has met those voters on the left.

But a wide range of polls have found Clinton losing votes to Libertarian Party presidential nominee Gary Johnson and Green Party nominee Jill Stein. A new CBS-YouGov poll of Ohio showed Clinton winning voters under 30 by 32 points. But that amounted to just 51 percent of the millennial vote and represented a six-point decline since the summer. According to the 2012 national exit poll, President Obama won 63 percent of Ohio voters under 30. That year, Obama never lost his narrow lead in an average of Ohio polls. This year, Clinton has lost that lead, despite Trump’s lack of a sustained ad campaign and Republican Gov. John Kasich’s ongoing refusal to endorse him.

A month ago Democrats were amused to see Johnson leading Trump along millennials. Now, Johnson’s support is being tackled like a crisis. Priorities USA Action, the pro-Clinton super PAC, stepped up its advertising to millennials last week, and the Sanders visit to Ohio was part of a several millennial-focused surrogate stops.

On Monday, Clinton plans to deliver a speech at Temple University in Philadelphia “laying out the stakes of November’s election for millennial voters,” according to her campaign. The speech will touch on the challenges facing young voters and will highlight several of her policy prescriptions, including a plan to make college available debt-free, aides said.

In a statement ahead of the speech, Clinton communications director Jennifer Palmieri acknowledged that Clinton has to go beyond making a case against Trump and offer reasons young voters should side with her.

“The millennial generation is a key voting bloc in this election, and it’s clear that the campaign must do more to earn their vote,” Palmieri said.

The Clinton campaign has 280 affiliates on campuses across the country; in June, it hired Sanders’s student organizer Kunoor Ojha to head up campus outreach. Priorities USA Action’s offerings include 15-second ads on YouTube that show millennials reacting with horror to Trump quotes about abortion, student debt and climate change. A $600,000 buy is running now in seven swing states, including Ohio.

In an interview before heading to Ohio, in the New York college town of New Paltz, Sanders said that millennial voters care about “the bloody issues,” waving his hand to indicate hundreds of students who had gathered to hear him discuss student debt and tax policy.

“I have been critical of the media because so much of the coverage deals with the gossip pertaining to the candidates,” Sanders said. “I’m not going to tell you that Hillary Clinton’s views are as progressive as mine. They’re not. But if you look at her views, they are pretty damn progressive. Making public colleges tuition-free — look, do you know what that would mean to these young people? Do you know what it would mean to kids in sixth grade right now?”

Convincing millennials

Sanders, who spent much of the summer finishing a book that will be released after the election, has begun to embrace a role as a millennial-whisperer. In interviews with The Washington Post and TV shows, he warned young voters that this was “not the year for a protest vote.” At one of his Ohio stops, in Akron, Sanders approached a group of protesters chanting “Bernie or bust,” gently chided them on their tone and invited them to listen to his speech.

They were not convinced.

“We really like Bernie, and we’re going to write him in,” said Nicole Baisden, 19, who came to the Akron rally wearing a pink dinosaur-shaped onesie. “When it really comes down to it, Trump’s a bigot and she’s a liar.”

Matt Marton, 22, listened to Sanders’s pitch in Akron, then rejoined Green Party activists who were driving to Kent State University to hand out fliers near the next speech. He had voted for Obama in 2012, then backed Sanders in the primary. Nothing Clinton had done had swayed him.

“If Hillary said, ‘Oh, yeah, I’m gonna do what Bernie wants,’ I wouldn’t be convinced,” Marton said. “If she gave a really passionate speech and said, ‘I was wrong,’ then I’d have to think about it.”

Johnson and Stein have capitalized on that mistrust. Johnson combines a cut-everything economic agenda with talk about drug decriminalization and ending foreign wars; Stein has suggested that all 43 million Americans who hold some kind of student debt could count on her for relief.

“Ours is the only campaign that will cancel that debt and bail out young people like the establishment bailed out Wall Street,” Stein said at a CNN-hosted town hall in August.

“We are investing tremendous time and resources on social media, videos and other communications efforts to connect with young people within the information sources they use,” said Joe Hunter, Johnson’s spokesman. “As we make that connection, we are finding a great deal of support.”

Rally by rally, Sanders was asking young voters not to go there. The socialist senator who celebrated his 75th birthday this month has favorable ratings in the mid-70s with voters under 30. After the Kent State speech, Noelle Elliott, 18, said she had not realized that Clinton had adopted a student debt plan until Sanders said so from the stage.

The fear of a Trump victory, something the Clinton campaign has been accused of relying on too heavily, was also starting to resonate. In conversations, many of the Kent State students at or near the rally said they did not think Trump had any chance of winning until the past week. Harold Horsley, 20, grabbed a Trump-Pence sign that he’d seen a mysterious man leave in a dining hall “as a motivational poster” while he kept wavering between Stein and Clinton.

Tommy Watral, 19, felt more of a responsibility to make converts. He showed up at the rally wearing an intentionally jarring shirt full of images of a smiling Sanders, jammed together as if a jar of them had spilled on the floor. It was a conversation-starter; anyone who talked to him would discover a former Sanders delegate to the Democratic convention who wanted to elect Clinton.

“I made a lot of friends my age at the convention who said, ‘Well, I guess it’s Hillary now,’ ” he said. “It wasn’t really us that held out. It was the older people who said, ‘No, Hillary’s the devil!’ We were like, ‘Guys, this isn’t your future you’re gambling with. This is ours.’ ”

A few miles away from the rally, the election was a much grimmer subject. The day that Sanders visited Kent State, the school’s football team played Monmouth University, a blowout tailor-made for a weekend when parents were visiting. As the team ran the ball, Chris Coffman, 22, said he was repulsed by Trump but preferred to choose a third party over Clinton.

“I don’t trust Hillary, what with all the email scandals,” he said. “And she’s late to everything. I mean, I was for gay marriage before she got around to being for it.”

Andrew Cosco, 19, said during a break in play that he’d vote for Obama if he was on the ballot. Since he won’t be, he was gravitating toward Trump. As a business major, he appreciated Trump’s experience and knew his wilder ideas “like the Wall” couldn’t come to pass.

“I guess I’ll watch the debates,” he said. “If she cleaned up some of what she’s doing, I could vote for Hillary. I just don’t know if that’s possible.”

John Wagner contributed to this report.