Trump, Clinton, and Obama will be there on Monday. The RCP average has Clinton up 4 in the state, but obviously it must be close to be getting this much attention. The Clinton campaign admits as much, per the CNN report linked above:

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.
“We’re taking that seriously,” he said.

Trump would get to 270 by holding the Romney states, and adding Florida, Ohio, Michigan, and the Maine Congressional district.