Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonWill the Horowitz report split the baby? Gabbard commemorates John Lennon's passing by singing 'Imagine' Bannon: Clinton waiting to enter 2020 race and 'save the Democratic Party from Michael Bloomberg' MORE slams Donald TrumpDonald John TrumpLawmakers prep ahead of impeachment hearing Democrats gear up for high-stakes Judiciary hearing Warren says she made almost M from legal work over past three decades MORE in a new ad for refusing to release his tax returns, attempting to draw a connection to the Republican nominee's ties to Russia.

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"Donald Trump's deep ties to Russia are a real problem," text reads in the beginning of the ad, titled "Investigation."  

"And a detailed investigation shows us why."

The ad then shows an ABC News reporter talking about an investigation that showed Trump's "real-estate fortunes have benefited greatly from Russian investors putting a tough U.S. policy with Russia in direct conflict with his bottom line."

The ad then pivots to speculation that Trump's refusal to release his tax returns is connected to his business dealings in Russia.

The ad argues that Trump's tax returns could reveal a lot about the GOP nominee.

"No more excuses, Donald," text says at the end of the ad.

"Release your hidden tax returns."

Trump has repeatedly declined to release his tax returns, despite calls from both Republican and Democratic leaders to do so.

He has argued that he is under audit and therefore can't release the documents. But the IRS has said nothing prevents Trump from releasing his returns.

Earlier this month, Donald Trump Jr. defended his father's decision not to release the documents, saying scrutiny of the tax returns would distract from Trump's campaign message.

Clinton often hits on Trump for declining to release his tax returns in an attempt to cast herself as the more transparent candidate.