Manafort resigned this week following reports questioning his work as a lobbyist for pro-Russian Ukrainian oligarchs.
“The hand of the Kremlin has been at work in this campaign for some time," Mook added. "It’s clear that they are supporting Donald Trump. But we now need Donald Trump to explain to us the extent to which the Kremlin is at the core of his own campaign.”
Mook explained that there are a “web of financial interests” that Trump has not disclosed, which has led to “real questions being raised about whether Donald Trump himself is just a puppet for the Kremlin in this race.”
When pushed by Stephanopoulos, Mook doubled down and called on Trump to disclose any ties he may have to Moscow.
Earlier this month, Ukrainian officials said Manafort’s name appeared on a list of "black accounts" made by ousted Ukraine President Victor Yanukovych, a known supporter of Putin. The payments totaled $12.7 million between 2007 and 2012. Manafort has denied receiving any off-the-books payments from pro-Russia Ukrainian groups, calling the notion that he received cash payments "unfounded, silly and nonsensical."
Suspicions about the Russian interest in Trump’s candidacy stem in part from the Republican businessman’s comments and long history of work with Russian business interests. Trump has long hinted that he would have a warm relationship with Putin.
“Putin hates us. He hates [Barack] Obama. He doesn’t hate [me],” Trump told reporters last fall. “I think he’d like me. I think I’d get along great with him, if you want to know the truth.”
Trump has also made repeated trips to Russia, including in 2013, when a Russian billionaire bought the rights to host the Miss Universe pageant.
The recent massive email hack on the Democratic National Committee, purportedly by Russian government agents, drew new and unwanted attention to the ties between Trump and Russian business interests.
ABC News’ Brian Ross, Matthew Mosk, Rhonda Schwartz and Megan Christie contributed to this report.