The Michigan Senate seat held by retiring Democratic Sen. Donald Riegle is up for grabs. If the GOP's Spence Abraham is ahead of Rep. Bob Carr by 6 points or more on election night, Barrett says, it could be an indicator of whether theRepublicans will come close to controlling either house of Congress. (In the House, they need 40 seats, in the Senate, 7.) "If a swing state like Michigan gives a candidate in an open race a significant majority, that's an indication the Republicans will do very well in a lot of places."ABRAHAM 36 (Detroit News) 42 (WJBK-TV)CARR 37 32"