10:23 p.m.At an event in Laramie, Wyo., Sanders used one word repeatedly to describe his victory in Wisconsin and the path that's led him there: momentum.
"Momentum is starting the campaign 60 to 70 points behind Sec. Clinton," he said. "Momentum is that within the last couple of weeks, there have been national polls which have had us one point up or one point down."
"We have now won seven out of eight of the last caucuses and primaries. And we have won almost all of them with overwhelming landslide numbers," he added.
10:19 p.m.Clinton tweeted to congratulate Sanders on his victory:
9:57 p.m.With results still coming in from across the state, here's a look at some of the key constituencies that helped contribute to Sanders' victory:
9:38 p.m. CBS News projects Bernie Sanders is the winner in Wisconsin.
As has been the pattern in previous Democratic primaries and caucuses, Sanders ran very strongly among younger voters while Clinton did better among those over 45. Sanders' lead among those under 45 is substantially larger than Clinton's margin among those over 45--he won among those under 30 by a 81- to-17 margin.
There was a significant gender gap as well: Clinton won 49 percent of women's votes, but only 37 percent of men's. Clinton also ran well ahead of Sanders among African American voters, but they were only about one out of 10 Democratic primary voters in Wisconsin. Sanders leads Clinton by almost 20 percent among white Democratic voters.
Clinton and Sanders ran about even among primary voters who said they think of themselves as Democrats, but Sanders had a big lead among independents voting in the Democratic primary.
More Democratic voters said that Clinton's policies are realistic (75 percent) than Sanders (65 percent). There is clearly an enthusiasm and inspirational gap between Sanders and Clinton: 54 percent of Sanders voters say they are excited about what he would do in office; 30 percent of Clinton voters said they were excited about what she would do in office. Fifty-nine percent of primary voters said Sanders inspires them more about the future of the country; 38 percent said that Clinton does.
On the other hand, Clinton is seen as the stronger candidate against Trump in November, by a 53-to-44 percent margin. Almost one in four of those who said that Clinton would be the strongest candidate against Trump still voted for Sanders.
9:00 p.m. Polls have closed in Wisconsin, and CBS News projects Bernie Sanders leads Hillary Clinton.
6:50 p.m. Here's where the race currently stands between Clinton and Sanders, both in terms of delegates and in terms of state victories so far:
5:52 p.m.As we wait for the polls to close in Wisconsin, here are some more highlights from the Democratic exit polls:
Democratic primary voters are divided over their expectations about life for the next generation of Americans: 33 percent think it will be better than life today, 38 percent think it will be worse, and 27 percent think it will be about the same. Sixty-two percent of Democratic primary voters say that their family's financial situation is holding steady, while 18 percent say they are falling behind and 19 percent say they are getting ahead financially.
Still, 31 percent say they are very worried about the direction of the nation's economy and 45 percent say they are somewhat worried.
5:15 p.m.Early exit polling is beginning to come in from Wisconsin. Here are a few highlights from the Democrats:
Jobs and the economy are the top concern of Democratic voters in Wisconsin, with 36 percent of them naming it as the most important issue facing the country. Second was income inequality, which 30 percent named as the most important issue.
As for the most important candidate trait, "honest and trustworthy" led with 32 percent, followed by someone with the "right experience" at 29 percent and someone who "cares about people" at 28 percent. Just 10 percent of Democratic voters said the most important candidate trait is someone who can "win in November."
5:00 p.m. Wisconsin voters are still heading to the polls in the first major primary of April, in a contest that could give Vermont Sen. Bernie Sanders another fresh
Polls will be open from 8 a.m. to 9 p.m. ET in the state's open primary. For Democrats, 86 delegates are at stake and they are allocated proportionally to candidates that meet a 15-percent threshold.
A CBS News Battleground Tracker Poll released Sunday found Sanders with a slight lead over Clinton, 49 percent to 47 percent in Wisconsin. According to CBS News' latest count, Clinton has 1,710 delegates compared with Sanders' 999.
Follow here for live updates throughout the night.
Stony Brook University political science professor Stanley Feldman contributed to this story.