“Jill Stein is a useful idiot for Russia. After parroting Kremlin talking points and being propped up by bad actors in 2016 she’s at it again,” DNC spokesman Matt Corridoni said in a statement to The Bulwark. “Jill Stein won’t become president, but her spoiler candidacy—that both the GOP and Putin have previously shown interest in—can help decide who wins. A vote for Stein is a vote for Trump.”
In addition, people act like she isn’t also the acting VP during this campaign. It would be extraordinarily problematic for the VP to actively undermine the policy of the president with whom they are serving even if their own presidential policy would be significantly different.