"It sounds like the only good thing Bernie can say about Hillary is that she's not Donald," Stein tweeted
, one of more than a dozen tweets she sent while Sanders and Clinton hosted a joint rally in New Hampshire where he officially endorsed the presumptive nominee. "That's what most of her supporters like about her."
Sanders and Clinton pitched themselves as joining forces to defeat Donald Trump, with Clinton declaring, "We are stronger together!"
Stein -- who has long been a vocal critic of Clinton -- disagreed, tweeting, "many Bernie hearts are breaking right now" and including an emoji of a breaking heart.
She added that the former secretary of state had "a major hand in creating the crises we face: disastrous crime bill, (the North American Free Trade Agreement), Wall St deregulation, destroying (the Aid to Families with Dependent Children program)."
Stein also wrote that she doesn't think Clinton believes in Sanders' mission -- tweeting that if Clinton did, she would choose him as her running mate.
"But she won't," Stein tweeted. "She doesn't offer full collaboration."
Later Tuesday, Stein penned an op-ed for The Hill
, calling her campaign plan B for Sanders' movement.
She compared Sanders' goals in his campaign to that of her own campaign, saying that within hours of Sanders endorsement, she got "thousands" more of supporters on Facebook.
"The consistent efforts of the Democratic Party to minimize, sideline, and sabotage the Sanders campaign are a wakeup call that we can't have a revolutionary campaign inside a counter-revolutionary party," she wrote.
In the op-ed, she pitched herself as the alternative solution for Sanders' supporters -- if they supported Sanders, they should support her because she has similar goals, she said.
"We cannot forget the triumph of Bernie's campaign, and the hunger of the public for an economy that works for working people, not just the billionaires," she wrote in the op-ed. "Let's keep the revolution going and build it into the powerful force for transformative change that it is becoming. Together we are unstoppable."