Sanders said he could help Democrats gain control of the Senate, however.
"It's going to be an uphill climb, but I think we have a shot," he said.
Clinton led Trump by 11 points in a similar poll last month.
Cuban said he's open to being Clinton's running mate, but he would have the same talk with Trump.
"I am going to be the nominee," Clinton said.
"I appreciate his openness to it," Clinton said of Cuban's suggestion of himself as VP.
The former candidate said Trump needs to work on his terrorism and healthcare proposals.
The Washington Post/ABC News poll shows them in a statistical tie.
"Crooked Hillary said that I want guns brought into the school classroom. Wrong!"
Huntsman said he expects Trump to grow over the remainder of the campaign.