Story highlights There's a a growing concern in Clinton's campaign that Michigan could land in Donald Trump's column

Trump's base of non-college-educated white voters has been a stronghold in the Midwest

(CNN) Hillary Clinton's campaign, in the waning days of the election, has turned its focus to Michigan, a state that months ago was considered safely in the Democratic nominee's column.

Clinton will make her second visit to Michigan in four days when she campaigns in Grand Rapids on Monday, the day before the election. Clinton has so far only spent four days campaigning in Michigan since the Democratic National Convention in July, a number that pales in comparison to the 13 days she has spent in Florida and the 10 days in Ohio.

President Barack Obama will also campaign in Ann Arbor -- home to the University of Michigan -- on Monday, looking to boost millennial voters the day before Election Day.

The trips reflect a growing concern that Michigan could land in Donald Trump's column on Tuesday.

"The numbers there have tightened," Robby Mook, Clinton's campaign manager, said Saturday, adding later that the tightening has been seen throughout the Midwest.

Read More