Donald Trump is going after Bernie Sanders for his expected endorsement of former Democratic primary rival Hillary Clinton.
Sanders, said Trump in a tweet, has "totally sold out to Crooked Hillary Clinton."
"Fans angry!" he added.
For weeks, Trump has been trying to woo Sanders backers frustrated with Washington politics by stressing his outsider candidacy.
Sanders has vowed to do everything he can to defeat Trump in November. Recent polling suggests that 85 percent of his supporters have said they plan to back Clinton.
The two are scheduled to hold the first joint campaign appearance in Portsmouth, N.H. on Tuesday.