With 91% of the vote counted, Vermont Senator Mr Sanders is polling 64%, while former Secretary of State Mrs Clinton has 36%.
In the Republican race, Marco Rubio easily won Puerto Rico’s primary, beating billionaire Donald Trump.
Mrs Clinton and Mr Trump remain overall leaders in the nomination campaigns.
Sunday night saw Mrs Clinton and Mr Sanders clash on a number of issues in a CNN-hosted debate in Michigan.
They traded accusations on economy and trade, with Mrs Clinton saying her rival voted against a bailout of the US car industry in 2009.
“I went with them. You did not. If everybody had voted the way he [Sanders] did, I believe the auto industry would have collapsed, taking four million jobs with it,” Mrs Clinton said.
Mr Sanders countered by saying: “I will be damned if it was the working people of this country who have to bail out the crooks on Wall Street.”
The BBC’s Kim Ghattas at the Clinton-Sanders debate
Bernie Sanders has rarely been so aggressive, losing his calm a few times on the debate stage. He attacked Hillary Clinton as a long-term supporter of free trade and talked about her friends in Wall Street who destroyed the economy.
She criticised him for not supporting the bailout that helped save the automobile industry, describing him as a one-issue candidate.
The debate took place in Flint, Michigan, which is facing a public health emergency because of lead-tainted water. The state will hold its primary on Tuesday.
Mr Sanders described the measures taken at the time as “the Wall Street bailout where some of your [Mrs Clinton’s] friends destroyed this economy”.
In Saturday’s round of voting, Mr Sanders took two states – Kansas and Nebraska – but Mrs Clinton maintained her Democratic front-runner status after a big victory in Louisiana.
‘One on one’ call
While the win in Puerto Rico – a US territory – will boost Florida Senator Mr Rubio’s campaign, it sends just 23 delegates to the Republican convention which nominates a presidential candidate.
Republican hopefuls need the votes of 1,237 delegates to get the nod for the presidential race proper.
Mr Rubio still trails well behind Mr Trump and Texas Senator Ted Cruz.
Speaking after wins in the Republican Kentucky caucuses and Louisiana primary vote on Saturday, Mr Trump told a news conference: “I would love to take on Ted Cruz one on one.”
“Marco Rubio had a very very bad night and personally I call for him to drop out of the race. I think it’s time now that he dropped out of the race. I really think so.”
Meanwhile, Texas Senator Mr Cruz – who won Republican caucuses in Kansas and Maine – said he believed that “as long as the field remains divided, it gives Donald an advantage”.
Source – bbc