In the Democratic race, Vermont Sen. Bernie Sanders holds a lead of 4 percentage points over former Secretary of State Hillary Clinton. Sanders takes 49 percent of likely Democratic voters, compared with 45 percent for Clinton. The results are within the poll's 6.3-percent margin of error.
Sanders has widened the gap with Clinton by 3 points from one month ago. A Marquette poll conducted in February found Sanders and Clinton neck-and-neck, 44 percent to 43 percent.
In the Republican race, Texas Sen. Ted Cruz is 10 points ahead of professional entertainer Donald Trump, with 40 percent of likely Republican voters, compared with Trump's 30 percent. Cruz has more than doubled his support since the February Marquette poll, which showed Trump leading Cruz 30 percent to 19 percent.
Other polls released this week also show the races competitive for Democrats and Republicans. The Marquette poll is the only recent survey using live phone calls to landlines and cellphones. Others were conducted with automated calling technology to landlines, or online.
Wisconsin's April 5 primary represents a critical point in the race for both party nominations.
For Sanders, it's an opportunity to dampen Clinton's delegate lead and pick up momentum before the New York primary two weeks later.
For Cruz, Wisconsin will be a litmus test to evaluate whether the anti-Trump movement has made progress toward the goal of a contested convention in July.
Wisconsin holds 86 delegates for Democrats that will be allocated proportionally, in addition to 10 superdelegates. Republicans will award 42 delegates, including 24 at the district level, 15 statewide and three to the overall state winner.
Marquette University surveyed 1,405 registered voters, including a subsample of 405 likely Democratic voters and 471 likely Republican voters. The margin of error is 6.3 percent among Democrats, and 5.8 percent among Republicans. The poll was conducted using live phone calls to landlines and cellphones.