Democratic presidential front-runner Hillary ClintonHillary Diane Rodham ClintonBiden: Probably '10 to 15 percent' of Americans 'are just not very good people' Mattis's Trump broadside underscores military tensions Mark Cuban says he's decided not to run for president MORE took a shot at rival Bernie SandersBernie SandersBiden formally clinches Democratic presidential nomination OVERNIGHT ENERGY: Trump signs order removing environmental reviews for major projects | New Trump air rule will limit future pollution regulations, critics say | DNC climate group calls for larger federal investment on climate than Biden plan Google: Chinese and Iranian hackers targeting Biden, Trump campaigns 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.