MACKINAC ISLAND, Mich. (AP) — Michigan Democrats are hoping to hold on to an open U.S. Senate seat as the party fights to win back a majority in Washington. But they first must settle their own arguments. The top three candidates competing for the nomination in the Aug. 4 primary are debating on Thursday at the state party’s annual policy conference. It is one of their first big opportunities to sharpen contrasts before a statewide audience. With the primary season wrapping up across the