Democratic presidential front-runner Hillary ClintonHillary Diane Rodham ClintonRepublican Nicole Malliotakis wins New York primary to challenge Max Rose Trump's evangelical approval dips, but remains high How Obama can win back millions of Trump voters for Biden MORE took a shot at rival Bernie SandersBernie SandersHickenlooper beats back progressive challenge in Colorado primary Progressive groups urge Biden to tap Warren as running mate Young Turks host says Elizabeth Warren should be Biden's VP pick MORE ahead of the Wisconsin primary on Tuesday, saying it’s “important” to choose a Democrat as the Democratic presidential nominee.

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"I … have been a proud Democrat all my adult life," Clinton said at a rally late Saturday in Eau Claire, Wis. "And I think that is kind of important if we are selecting someone to be the Democratic nominee of the Democratic Party."

Clinton and Sanders, an independent Vermont senator who caucuses with Democrats, are in a tight race in Wisconsin. Sanders holds a slight edge, 46.8 percent to 44.5 percent, according to a RealClearPolitics average of polls.

Sanders campaign spokesman Michael Briggs told The Wall Street Journal that Sanders is a “a lifelong progressive who has caucused with the Democrats since he came to Congress.”

Briggs also noted Sanders’s efforts to elect President Obama and work to raise funds for Democratic candidates.

“And, oh yeah, he didn’t support Barry Goldwater for president,” Briggs added in a shot at Clinton.