So there could be a decent amount of Clinton-Trump voters in the Midwest? Who are these people? Clinton was an abysmal candidate for the Midwest, hardly campaigned in Michigan and Wisconsin, and lost them by less than 1%. The Democratic nominee will heavily target these states in 2020, and likely won't be as unpopular as Clinton. How could Trump realistically expand his margin there?
The inverse is more true, that Obama-Trump voters arguably tipped the Rust Belt to Trump. They came back to the Dems in 2018. There is zero evidence of Trump expanding his base with new voters or Obama-Trump-Dem2018 voters once again returning to Trump en masse (one of which Trump needs to win MI/PA/WI again). In fact, it seems quite clear--the Siena outlier aside--that the Obama-Trump voters took a shot on Trump the Outsider and do not at all feel compelled to make that choice again, and they will either stay home or come home to the Dems.