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. “We’re taking that seriously,” he said. A recently released Detroit Free Press poll found that Clinton is up 4 points in Michigan, 42% to Trump’s 38%. Clinton’s campaign has leaned heavily on its ground game to turn out voters early in key swing states, trying to “bank” votes before Election Day. The campaign has particularly seen early voting gains in Florida, Nevada and North Carolina. Some aides are quietly confident that, given the turnout figures in Nevada so far, the state could already be in Clinton’s column. Most voters in Michigan, though, cast their ballots on Election Day, adding a level of volatility to the state that worries Clinton aides. Michigan has been a thorn in Clinton’s side throughout the 2016 campaign. Clinton lost to Vermont Sen. Bernie Sanders in the Wolverine State’s March 8 primary after campaigning for the better part of a week in the state. Aides at the time chalked up the loss to the state’s predominantly white population, and Clinton bounced back to win the nomination, but the loss was arguably the most stinging of Clinton’s primary run. Mook dismissed the idea that reluctant Sanders supporters were the reason for Clinton’s current concerns in Michigan. But Trump’s base of non-college-educated white voters has been a stronghold in the Midwest throughout the 2016 race. He dominated the states in the Republican primary, and while more demographically diverse states like Colorado and Virginia have tilted toward Clinton, Midwest battlegrounds like Ohio have held steady for the Republican nominee. “From what I have seen, this is a natural tightening of the race. If anything, in the last few days, we have seen things improve a little bit,” Mook said. In addition to Michigan on Monday, Clinton will campaign with Obama, First Lady Michelle Obama and former President Bill Clinton in Philadelphia and in Raleigh, North Carolina. The Raleigh event, said Jennifer Palmieri, Clinton’s spokeswoman, will be near midnight. Clinton’s top aides rejected the idea that the tightening in Michigan was caused by FBI Director James Comey’s announcement to Congress about a review of emails potentially related to Clinton’s email server. “We saw the race begin to tighten before then,” Mook said, adding that the letter and probe knocked Clinton off message.