Voters have been making their way to the polls in Michigan on Tuesday, in a presidential primary that will test how much Joe Biden and Donald Trump should be worried about winning key groups of voters for the general election in the critical swing state.Both face challenges within their respective parties. After underperforming in the polls and struggling with suburban and college-educated Republican voters in earlier primaries, Trump’s campaign in Michigan is dealing with a state Republican party whose local leaders have been embroiled in an ugly factional dispute, while Biden faces a…