'Fans angry!': Donald Trump criticizes Bernie Sanders for 'selling out' and campaigning with Hillary Clinton

Donald Trump railed on Bernie Sanders Tuesday morning for "selling out" ahead of the Vermont senator's joint rally with Hillary Clinton where it's widely expected he will endorse the presumptive Democratic nominee.

"Bernie Sanders, who has lost most of his leverage, has totally sold out to Crooked Hillary Clinton," Trump tweeted. "He will endorse her today - fans angry!"

He added that he's "somewhat surprised" Sanders decided to endorse Clinton more than a month after the former secretary of state clinched the party's nomination.

"I am somewhat surprised that Bernie Sanders was not true to himself and his supporters," Trump posted. "They are not happy that he is selling out!"

The presumptive Republican nominee has made repeated pleas for Sanders supporters to come over to his camp instead of Clinton's since he clinched his party's nomination in May. But in a late-June ABC News/Washington Post poll, just 8% of Sanders supporters said they'd vote for the Manhattan billionaire, down from 20% in May.

Sanders joins Clinton for the rally Tuesday in New Hampshire. The two candidates have been recently taking steps toward unity, as Sanders received some concessions on the Democratic Party platform ahead of this month's convention in Philadelphia.