Story highlights "Trump's hiding something, and the only question is what," Kaine said

Kaine has stepped up his criticism of Trump in recent days

Erie, Pennsylvania (CNN) Sen. Tim Kaine leaned further into the role of vice presidential attack dog Tuesday, suggesting Donald Trump wouldn't stand up to a potential Russian hack of the American election.

The Virginia Democrat also hit Trump for not releasing his tax returns or disclosing business ties to international entities, suggesting the billionaire is "hiding something."

"When candidates are on the up and up, they have no problem disclosing this information," Kaine said at a Pennsylvania organizing event here on the shore of Lake Erie. "Trump's hiding something, and the only question is what."

Hillary Clinton's running mate cast doubt on Trump's intentions, accusing him of having a "cozy" relationship with Russia.

"Don't you think the American voters deserve at least to get to the bottom of why Trump's relationship with Russia is so cozy?" he asked. "We have no idea how he might stand to profit from Russia or what they might be holding over him."

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