Story highlights Stein wrote a series of tweets lamenting Sanders' endorsement of Clinton

She says she doesn't believe the presumptive Democratic nominee will fully cooperate with him

Washington (CNN) Likely Green Party presidential candidate Jill Stein lamented Bernie Sanders' endorsement of Hillary Clinton Tuesday, tweeting, "many Bernie hearts are breaking right now."

"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."

It sounds like the only good thing Bernie can say about Hillary is that she's not Donald. That's what most of her supporters like about her. — Dr. Jill Stein (@DrJillStein) July 12, 2016

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.

Read More