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.