Some of Bernie Sanders’ most loyal backers have turned into his biggest bashers on the heels of his Hillary Clinton endorsement.

The Vermont senator, who slammed Clinton repeatedly during the presidential primary campaign, offered his unwavering support to the presumptive Democratic nominee at a rally in New Hampshire Tuesday. 

“Hillary Clinton will make a great president and I am proud to stand with her today,” he said.

What followed was an avalanche of angry tweets, blogs and other social media posts from those who had been feeling the 'Bern' -- and now just feel burned.

In New York, Monroe County Sanders activist Kevin Sweeney told the Democrat & Chronicle he's shifting his donations to Green Party candidate Jill Stein. "A lot of Bernie supporters are making $27 donations to Jill Stein's campaign today," he said. 

Others were more direct, as the hashtag #SelloutSanders and others took off on Twitter: 

The subreddit thread “Sanders For President,” which has more than 230,000 followers, also reacted strongly to the endorsement.

Surfer808 wrote, “Sorry Bernie, I love you but even with your endorsement I will not vote for Hilary (sic).”

_shane posted a picture of Bernie bumper stickers and wrote, “Getting this in the mail today was like getting kicked when you’re down.”

A user under the name Ruscer wrote, “We’re left with a race between two liars. One who just recently was caught in a web of lies that she slithered her way out of using her power and influence, and the other a racist demagogue.”

In an interview with CBS News, Sanders responded to the frustration.

“I don’t think most of them are calling me a sell-out,” he said. “In the real world, you’ve got to make difficult choices.”

Donald Trump, the presumptive Republican presidential nominee, jumped in on the action.

He tweeted, “Bernie Sanders endorsing Crooked Hillary Clinton is like Occupy Wall Street endorsing Goldman Sachs. “

But not everyone is hating on Sanders.  

“Everyone calling Bernie a 'sell out' or 'traitor' – hope you’re heeding his call and getting involved in local government,” Jessica Sawyer wrote on Facebook. “I understand your frustration but it would be better utilized in a productive manner, instead of name calling on the interwebs.”

Stephen Gomez, the bassist for the band The Summer Set, tweeted, “I applaud Bernie on his endorsement of Hillary, and it was an honor to be part of his campaign. He has truly changed the face of US politics.”