"Let me be very clear. It is imperative that Jon Ossoff be elected congressman from Georgia's 6th District and that Democrats take back the U.S. House," Sanders said in the statement released by his political office.

"I applaud the energy and grassroots activism in Jon's campaign. His victory would be an important step forward in fighting back against Trump's reactionary agenda."

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While the statement puts Sanders behind Ossoff in his bid against Republican Karen Handel, Sanders didn't reverse earlier comments about Ossoff that drew ire from national Democrats.

When asked Wednesday whether Ossoff is a progressive, Sanders said to the Wall Street Journal : "I don't know." "If you run as a Democrat, you're a Democrat. ... Some Democrats are progressive and some Democrats are not," he said. The comment raised eyebrows among some Democrats who were concerned that the comments could open old wounds from the Democratic presidential primary and hurt party unity around Ossoff, whose race has become the focus of national attention as a potential referendum on President Trump. But while progressive groups like the Progressive Change Campaign Committee and the liberal blog Daily Kos backed Ossoff, other Democrats warned that hugging the left too tightly could hurt Ossoff with moderate Republicans who could help him win in the right-leaning district.

Ossoff advanced to the runoff after winning 48 percent of the vote in a Tuesday special election.

Democrats are hoping that flipping the red seat could act as a warning to Republicans about the safety of GOP-held districts now that Trump leads the party. By failing to receive more than 50 percent of the vote on Tuesday, though, Ossoff failed to win the contest outright and now heads to the runoff.