Liberal groups are increasingly worried that Libertarian presidential candidate Gary Johnson is presenting a bigger threat to Democratic candidate Hillary Clinton than to Republican opponent Donald Trump.

Their main fear is that Johnson, formerly governor of New Mexico, is attracting fans of Clinton's Democratic primary rival, Sen. Bernie Sanders of Vermont. A group calling itself the Democratic Coalition Against Trump and billionaire environmentalist Tom Steyer's NextGen Climate have begun running web ads geared to scaring those voters away from Johnson.

Nate Lerner, executive director of the Democratic Coalition Against Trump, said his group's concern was that a lot of Sanders' voters, particularly those under 30, weren't aware of Johnson's platform aside from left-leaning issues such as marijuana legalization.

"I get the appeal: He is a cool, laid-back kind of guy," Lerner said. "He has a lot of good ideas."

The web ads highlight Johnson's other free-market beliefs, with a particular emphasis on various comments Johnson has made downplaying fears of climate change.

"Climate change will cost millennials over $8 trillion," said one of NextGen Climate's ads before cutting to a montage of clips of Johnson fumbling on the issue. It concludes: "Vote on Nov. 8 for someone who gets it."

Democratic Coalition Against Trump's message was even more direct: "If you support Bernie Sanders, you don't support Gary Johnson," it says in bold letters before cutting to series of clips of Johnson explaining his positions and Sanders doing things like rolling his eyes.

President and Michelle Obama have been targeting the millennials on the campaign trail, telling them that voting for Trump would hurt Clinton.

"If you don't vote, that's a vote for Trump," Obama said in an interview on the Steve Harvey Morning Show. "If you vote for a third-party candidate who's got no chance to win, that's a vote for Trump."

The groups have expressed little concern that Green Party candidate Jill Stein would draw votes away from Clinton, although, as a hard-left liberal, Stein in theory would have much more appeal to Sanders' fans than Johnson.

"I don't think she is as much of a threat. She's certainly not as likeable as Gary Johnson," Lerner said.

Libertarian Party candidates are generally presumed to get more support from people who would otherwise vote Republican than Democrat. Liberal activists generally therefore don't view them as a threat. Lerner said his group was "really excited" when Johnson announced his 2016 bid because they thought he would hurt the GOP.

But that may not be the case in the 2016 election thanks to Sanders' primary challenge, which engaged many millennial voters, a group that overall leans left. A recent Economist/YouGov poll found that while Johnson was getting 5 percent of the vote overall, he was getting 13 percent of the under-30 vote, his strongest showing among all age groups. Another 32 percent of under 30 voters said they would consider backing him.

The same poll showed that Clinton was getting 53 percent of voters 30 and under, Trump 23 percent and Stein just 5 percent. Clinton also appears to be close to a ceiling on her support from that group. Just 8 percent of those who aren't already committed to voting for her said they would consider it.

Exit polls in the 2012 election found by comparison that President Obama got 60 percent of voters 29 and younger, indicating that Clinton is under-performing in that group, though so is Trump. Republican candidate Mitt Romney got 37 percent of that group that year.

Johnson also ran as the Libertarian Party candidate in that election, getting just 1 percent of the vote. The current polls suggest he has managed to significantly expand his appeal. Third-party candidates typically do not do as well on Election Day as they do in polls leading up to it, however.

Nevertheless, Clinton's slack support among millennials and Johnson's potential appeal to them has liberals worried. Lerner said he believed that Johnson was even deliberately pitching himself to liberals more than conservatives in recent weeks.

Part of the problem he conceded was that liberals just aren't super-excited about Clinton the way they were for Obama. "We're not coming at this from a pro-Hillary standpoint. We are coming at this from an anti-Trump perspective," Lerner said.