Hillary Clinton holds a 9-point lead over Bernie Sanders with 51 percent support heading into the important Wisconsin primary Tuesday. However, the open primary format in Wisconsin may give Sanders an advantage over Clinton as he continues to do extremely well with independents who lean toward the Democratic Party.

These results are according to the latest from the NBC News|SurveyMonkey Weekly Election Tracking poll conducted online from March 28 to April 3 of 14,071 adults aged 18 and over.

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Wisconsin utilizes an open primary format, which means any registered voter can participate in either the Democratic or Republican contest regardless of their party identification. In the 2008 Democratic primary, 28 percent of Wisconsin primary voters identified as independent, and 62 percent identified as Democrat, according to the NBC News exit polls.

In this week’s national poll, Sanders is beating Clinton by more than 20 points among independents who lean toward the Democratic Party and are thus are more likely to vote in a Democratic primary where allowed.

While Sanders is the clear favorite among independents who lean toward the Democratic Party, those who identify solidly as Democrat support Clinton over Sanders by a nearly identical margin — 56 percent to 37 percent.

The key to the remaining contests in the presidential primaries is the size of particular groups in individual states as Clinton and Sanders have done very well with distinct segments of the electorate. Our national poll numbers show that while Clinton is clearly ahead in the head-to-head with Sanders, he remains competitive with her with a few key groups in Tuesday’s contest in Wisconsin. Among white voters — who made up 87 percent of the electorate in Wisconsin’s 2008 Democratic primary — Sanders and Clinton are currently tied.

The open question in every election is which groups will turn out. That will be critical for Tuesday’s contest for both candidates.

The NBC News|SurveyMonkey Weekly Election Tracking poll was conducted online March 28 through April 3, 2016 among a national sample of 14,071 adults aged 18 and over, including 12,116 who say they are registered to vote. Respondents for this non-probability survey were selected from the nearly three million people who take surveys on the SurveyMonkey platform each day. Results have an error estimate of plus or minus 1.1 percentage points. For full results and methodology for this weekly tracking poll, please click here.