Duke called Trump's campaign "a great opportunity" for "European American" interests.
Trump "has no moral or ethical grounding," Sen. Chris Murphy said.
"He’s going to win if we see more of the good Trump and less of the bad Trump."
Trump also says he's had "really nice conversations" with Marco Rubio.
The New Mexico governor had been considered a potential Trump running mate.
The Senate minority leader showed he will be a vocal critic of the presumptive Republican nominee.
Timothy Cama 05/05/16 02:06 PM EDT
The group's president slammed Clinton for supporting "job-killing Obama policies."
"I don't think he can lend his credibility to Hillary's incredibility," Nader said.
Mnuchin has reportedly given more than $125,000 to candidates since 1998.
Dole is the only former GOP presidential nominee slated to attend the convention.