MIAMI -- Even before Tuesday's stunning results from Michigan came rolling in, Sen. Bernie Sanders showed he had some fight left in him.

The Democratic White House hopeful announced he was filing a federal lawsuit in a bid to force Ohio to allow 17-year-olds to vote in next week's presidential primary there if they will be 18 by the time of the general election.

"This campaign is very proud of the fact that we are bringing many, many people into the political process," Sanders told reporters as he prepared to board his charter jet in Detroit en route here. "We want to continue to see that ... Unfortunately, in the state of Ohio, there is an effort on the part of the secretary of state to do exactly the opposite."

The Republican secretary of state, Jon Husted said he welcomes the lawsuit "because the law is crystal clear" and on his side.

Ohio is among the five states with Democratic primaries next Tuesday, and Sanders is hoping to replicate his success in neighbouring Michigan, where he scored a narrow upset of Hillary Clinton. Young voters continued to be a bedrock of Sanders' coalition in Michigan. He won those ages 18 to 29 by a margin of 81% to 18% on Tuesday, according to exit polls reported by CNN.