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Cruz got 40 percent support in the poll released by the Marquette University Law School today. Trump earned 30 percent and John Kasich earned 21 support.
The Wisconsin primary, slated for April 5, marks the next major presidential contest in the election season.
Democratic delegates are allocated proportionally. The GOP contest is winner-takes-most, with the potential to become winner-take-all if the candidate picks up all eight congressional districts.
Trump was the least popular candidate in this poll, with seven in 10 registered voters holding an unfavorable view. Six in 10 saw Clinton unfavorably with half seeing Cruz mostly negatively.
In a head-to-head preview of November elections, the poll shows Clinton would defeat Trump by 10 percentage points, but Cruz ties and Kasich defeats her by 9 points.