Sanders' victory in Michigan revealed Clinton's possible weakness in the Rust Belt states that might be an opening for Sanders to close the 200-delegate gap between them.
Clinton maintained her strong support among African-American voters with her big win in Mississippi. Sanders won 1 out of every 10 black votes in Mississippi as Clinton continued to dominate in the Deep South.
Clinton won easily with 82 percent of the vote to Sanders' 16 percent there.
But that changed slightly with Michigan, where Sanders cut into her support with Clinton winning 65 percent of the African-American votes there.
Clinton saw up to a 20-point lead in Michigan collapse in just days, with Sanders ultimately slipping past her 50 percent to 48 percent.
"I am grateful to the people of Michigan for defying the pundits and pollsters and giving us their support," Sanders said in a statement.
Polls underestimated the number of Republicans and independents who would come out to vote for Sanders or against Clinton. Exit polls revealed 69 percent of Democratic primary voters identified as Democrats. Another 28 percent said they were independents, and 3 percent were Republicans.
Sanders beat Clinton big among independents, 70 percent to 28 percent, which was more than enough to offset Clinton's 16-point advantage with Democrats.
"Thank you, Mississippi!" Clinton tweeted Tuesday night.
If the projection holds true, Clinton is expected to win 40 delegates from Mississippi.
According to earlier polling data, the two candidates were split among voters on many issues.
Roughly 60 percent said they believe Clinton to be honest and trustworthy, versus about 80 percent for Sanders, Politico reported. And 60 percent said honesty or empathy is the most important attribute for a candidate.
"Today, Michigan and Mississippi head to the polls. Here's how we can bring home two more wins," Clinton tweeted earlier Tuesday, linking to a campaign page encouraging supporters to spread her message on social media channels.
"The early states are behind us. Super Tuesday has come and gone," the page stated. "Hillary Clinton's leading Bernie Sanders in votes, delegates and states. But winning isn't the same as won -- and Hillary needs your help to get out the vote in Michigan and Mississippi today."
Clinton also shared a campaign "hotline" voters can call if they have any questions about her stance on issues.
"Ready for a political revolution? Get out and vote!" Sanders' campaign tweeted Tuesday.
"Change cannot take place without political participation," another tweet read, encouraging Mississippi and Michigan voters to visit the polls. "What this campaign is about is bringing people together. Michigan and Mississippi: please go vote today."
About 70 percent of voters in the two states said they would be satisfied if Sanders won the Democratic nomination -- compared to roughly two-thirds for Clinton.