Sen. Al Franken, D-Minn., pushed FBI Director James Comey on Wednesday to investigate President Trump's unreleased tax returns as part of its probe into whether Russia interfered with the U.S. election, and whether Trump's team colluded with Russian officials along the way.

At a Senate Judiciary Committee hearing, Franken cited numerous sources that indicated one of the ways Russia tried to broker its influence outside of their country was by cultivating deep ties with foreign businessmen and women.

Franken argued that to understand if Trump was vulnerable to corruption by Russia, "we would have to understand his financial situation. We'd have to know whether or not he has money tied up in Russia or obligations to Russian entities, do you agree?"

"Director Comey, would President Trump's tax returns be material to such an investigation?" he asked.

But Comey refused to answer.

"That's not something, senator, that I'm going to answer," Comey replied. And when Franken asked if the FBI has access to Trump's tax returns, Comey said, "I'd have to give you the same answer."

By pressing for the tax returns, Democrats hope to kill two birds with one stone: Finding new avenues for investigation between any potential collusion between the Trump campaign and the Russian meddling in the 2016 elections, and potentially finding embarrassing information about how Trump managed his taxes in the years leading up to his election.