Bernie Sanders has beaten Hillary Clinton in the Maine caucuses, the latest contest in the battle to be the Democratic presidential candidate.

With 91% of the vote counted, Vermont Senator Sanders is polling 64%, while former Secretary of State Clinton has 36%.

In the Republican race, Marco Rubio easily won Puerto Rico's primary, beating billionaire Donald Trump.

Clinton and Trump remain overall leaders in the nomination campaigns.

Sunday night saw Clinton and Sanders clash on issues including economic policy in a CNN-hosted debate in Flint, Michigan.

In Saturday's round of voting,  Sanders took two states - Kansas and Nebraska - but 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 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.

Rubio still trails Donald Trump and Texas Senator Ted Cruz.

Speaking after wins in the Republican Kentucky caucuses and Louisiana primary election on Saturday, 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 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".