"We think that the positions of Donald Trump, the man running for president, and Gov. Pence are in the mainstream of America," Manafort said on "Fox News Sunday."

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"And we’re confident that the combined Trump/Pence ticket is going to be very appealing to moderates, conservatives and, frankly, liberals who were supporting Bernie Sanders because of his campaign against the rigged system, the rigged Wall Street system, the rigged political system, the rigged trade system."

Sanders endorsed Clinton earlier this month.

Trump slammed Sanders after the endorsement and invited his supporters to back the Trump campaign.

"The messages of change that Trump campaigned on," Manafort said, "are those messages that were popular to Sanders supporters and we think will be an appeal to us."